Tuesday 14 March 2017

CZE T251 FM Broadcast transmitter REVIEW

This is an overlook and general impressions review of the Chinese made CZE T251 FM Broadcast transmitter. I ordered mine in 2017 out of China from eBay seller 'Thanksbuyer-hobby' for $219,- including shipping. It took 11 days to arrive which is fast!

Update 1st of November 2020.  Before we start: I haven't been working with FM transmitters in the last 2 years but I had a look lately and it seems they have gone up in price quite a bit. I could also find only one listing of this particular transmitter on eBay. Just so you know, they are getting rarer and are probably being replaced by newer models but I can not give you any opinion on those. As I said I no longer occupy myself with transmitters. It's all synthesizers now =) Therefore the comments for this article are now also closed. The new GD-20155-ALL 15 Watt FM transmitter using the BH1414 chip looks like a good one to buy though.
BUT KEEP IN MIND: Chinese equipment is okay as long as it works but when it goes wrong and you need to refer to customer service you're in for some big disappointments.

Normal operation:
You switch on by pressing the on/off button at the back. Then you get either the standby screen or the transmit screen, depending on how the transmitter was set when it was last switched off. If you press the front dial for more than one second the transmitter goes into sleep mode and won't transmit. The button will light up red. A simple short press of the front dial button awakes it and powers it up. The button now lights up blue.
Here are some pictures to illustrate this:

In transmit mode the display shows, from top left to right: the frequency it's transmitting on, the audio volume (goes from 0 to 79, you can see in the picture above the audio is set to the maximum 79) and whether it's stereo or mono (one or two speakers displayed) and the microphone level (also from 0 to 79, M00 means microphone level is set at minimum). The row underneath shows the RF power output, the SWR reading and the temperature of the power transistor. When transmitting, turning the front dial regulates the output audio volume of the transmission. You can access the menu while transmitting and change settings without the transmission being disturbed but if you change the RF frequency, the transmission will stop.

The Menu consists of the following items:
1 - Frequency set
2 - Audio Volume
3 - Microphone Volume
4 - RF Power set
5 - Stereo or Mono
6 - Temperature Alarm set
7 - SWR Alarm set
8 - Mute
9 - Exit Menu
If you turn the dial clockwise you go through the menu from item 1 to 9. If you turn counter-clockwise you go through the menu from item 9 to 1.

Things to be aware of:
First thing to be aware of when you buy this transmitter without a power supply is that it needs a 2.5mm inner diameter/5.5mm outer diameter DC barrel connector. Not the standard 2.1/5.5mm ones that are used with, for instance, the CZE/7C. The correct procedure to switch this unit off is to first press the button at the front to put it in sleep mode. If the fan is running when you switch off like this, it will keep running for a short while (about 20 seconds, give or take) and then, when the fan stops running, you can switch the unit off by pressing the on/off button at the back. If you switch off following this procedure, the unit will always start up in sleep mode. This prevents it from immediately transmitting when switched on. A simple short press of the front dial button will then start the transmission. The RF power will start rising smoothly to the set output power and will reach that power level within a maximum of approximately 10 seconds.
Here are two pictures of the backside:

Also something to be aware of is that this unit has as standard an N-type RF output socket (female). I screwed an adapter plug (N-male to SO239) onto the antenna output socket to change it into an 'SO-239' because I only use 'PL-259' plugs on my coax.
If you look closely at the top two pictures you can see, I made two little extensions (from some Ø15mm copper tubing) for the front feet so the transmitter stands a little higher. The air intake is underneath and at the front so it's best to put the transmitter a bit higher on its feet to give it some space to breathe in air. If you don't do this the front panel almost touches the surface with the main body clearance being 4.5mm. On YouTube I've seen a few other users of this unit do the same thing. You don't HAVE to do this as long as the unit stands on a smooth flat surface.
Before owning this transmitter I had used the CZE-7C, 7 Watt transmitter and I must say I liked it a lot. Even-though many reviews complain about the sound quality of the CZE 7C, on my car radio it sounded pretty good but the T251 sounds even better! Anyway, it's good to have a CZE 7C handy. In my case I use it to test the bandwidth of my antenna and of Low Pass filters I build etc. So I sort of use it as a signal generator, because you can change frequency real easy whilst transmitting. That is something the T251 will not do! If you go into the menu to change the frequency, the RF stage switches off. Btw, I noticed that if you put the T251 in sleep mode after it has been transmitting it still transmits a very weak signal. It can't be more than a few micro Watts but I can still hear the music buried in noise on my radio. Probably the FM chip that still gets power from electrolytic-capacitors that haven't discharged yet.
The FM chip:
I used to think this transmitter was equipped with the BH1414K FM-chip but I inquired with the seller and they informed me that it has the BH1415F chip inside. This was a disappointment for me, because for the price I thought it would have the much better BH1414K, but still, it does a pretty good job. Of course it's not only the chip itself that is important. It's also the circuitry around it, and there is a distinct difference between the signal from this transmitter and the signal from the cheaper ones which also use the BH1415F like the CZE 7C. That's mostly due to the end-stage with it's filters and power transistor. The RD30HVF1 mosfet is an excellent and robust transistor for the end stage. However, it is noted in the comments below that the BH1415F chip can cause some overload issues that result in stations near to the transmitting frequency being overpowered by the CZE's signal for about a maximum of half a mile around the transmitter location. So be aware of that. It's not perfect (obviously).
Frequency stability is rock solid and stays well within the advertised maximum deviation of 10Hz.
Of course you can analyse the signal from this transmitter until you're blue in the face and it's never going to be as good as a professional Broadcast transmitter but that is reflected in the price.
Here is a picture of the output sinewave at 5 Watt RF power on an oscilloscope. As you can see a nice clean signal.

Pre-emphasis delay is 50µSec and can not be changed in the menu. So this transmitter is not aimed at the American market.  You will just have to contact the seller and ask if they have units with 75µSec pre-emphasis delay if you live in the United States. USA Stock is regularly promoted from different sellers on eBay. (I had a link here to some USA stock on eBay but the listing has ended.)
This transmitter has an RS232 connection at the back, underneath the N-Connector but don't think you can control this unit with your PC. That connection is only for debugging and firmware updates.
Don't assume either that the extra 18 Watts of power compared to the CZE 7C is going to increase your range significantly with the same antenna setup. It's not. You want bigger range? Put your antenna up higher. This set gives me just a little bit more range than the CZE 7C with the same open dipole antenna that I use and the signal is a bit stronger within the normal range but it's only a very small improvement. Believe me, like with an audio system, the quality of the speakers is everything, so it is with antennas and especially their height. (I address the point of 'range vs power' at the bottom of this review also.) As a rule, to double your range with the same antenna set-up, you need a 10 fold increase in output power!!

A closer look at the signal:
Alas I don't have expensive spectrum analyzers etc so here's a few screenshots of the signal from my Software Defined Radio (SDR) on my computer (click on the picture to get full screen view):

Stereo transmit signal at 25 Watts:

This is the signal from a professional local FM radio station whose transmitter and antenna are located just 100 meters from my house. Compare this signal to the previous picture of the T251. Pretty similar right?

Btw, if you look at the signal to noise ratio (SNR) you can see that it is a bit lower on the T251 than on the professional transmitter but that changes with the type of music. If there's a low volume bit in the music the SNR on the T251 can go as high as 37dB. Btw, that is SNR measured with this SDR receiver. That's not exactly a precision instrument. The real Signal to noise ratio is advertised as being equal to or above 70dB. And in fact, CubicSDR software indicates a SNR between 65 and 70dB so that is excellent. These numbers I mentioned are for the stereo transmissions. When you transmit in mono, the signal to noise ratio will be even higher and the mono transmissions also have a longer range than the stereo transmissions.

Mono signal:

Stereo signal but no audio input:

Signal comparison between the CZE-7C (7 Watt transmitter) and the CZE-T251 with the same song playing, both in stereo and both audio volumes and output power set to the same level. Note the difference in deviation beyond the 200kHz bandwidth and the more slender signal of the T251:

This is the CZE-7C. You can see in the waterfall display that the audio bleeds over the 200kHz bandwidth limit, causing disruption on the adjacent frequencies. You can also see that the signal is quite wide:

This is the CZE-T251, here you can see the audio stays nice and tight within the 200kHz bandwidth limit, like a good transmitter should do. The signal is also much more slender than that of the 7C:

Here are some screenshots done with "CubicSDR" software:

Signal at 25Watt without modulation. You can see the 19kHz stereo pilot-tone and more in the upper right audio spectrum graph:

Signal with normal music. Again note the audio graph in the upper right-hand corner:

This is an illustration of how the audio signal produced by an FM stereo transmitter is build-up and how it occupies the frequency spectrum:

You can find more on the technicalities of FM Broadcasting on Wikipedia.

More observations about the T251:
The transmitter is very well built. It's all thick Aluminium. There's no plastic on it anywhere except for the on off switch at the back. Even the dial knob at the front is made from Aluminium. It is a lot smaller in size then I expected from the pictures on eBay though. Be prepared for that. The size is: front-panel width: 173mm (6.81") x front-panel height: 58mm (2.28") x depth: 210mm (8.27"). Main-body width: 167mm (6.57"). Main-body height: 53mm (2.09")
The power amplifier part (end stage) is mounted to a big heat sink with ribs that go all the way to the side of the case and with a fan attached to one side which blows air over it. This is more than adequate to keep the temperature down. (More on the fan in the item below.)
The transmitter is equipped with a temperature alarm. If the temperature gets higher than the alarm setting the unit will stop transmitting and an alarm will sound.  I have set mine to 50°C. The power transistor can easily take 120°C according to the spec sheet but the fan keeps the transistor at a maximum of 39°C so if the temperature rises above that, then it's obvious that there's something wrong and in that case the sooner the unit switches off, the better.
The unit also has a build in SWR meter with a programmable SWR alarm setting. If the alarm is triggered the unit stops transmitting immediately and an alarm will sound continuously until you switch the unit off. If you then switch on again the RF power output has automatically changed to 1 Watt. This is obviously a safety measure in case there's still something wrong with your antenna system. If all is well, you can go into the menu and set the power to your desired value. The SWR meter is a bit optimistic. It says 1.0 when it is really 1.1 but that's no problem in normal use.
The power transistor used is the RD30HVF1 mosfet rated at 30 Watts. It has a typical efficiency of 60% (meaning 60% of the energy put in is transferred to RF power, the rest to heat) and can take an SWR mismatch of 20 to 1 without being destroyed (!!!) The temperature sensor is mounted near the mosfet and is of the LM35 centigrade type. Both the sensor and the power transistor are generously covered in a heatsink compound as you can see in the pictures in the link below. In use with a voltage of 12.55 Volts I draw about 3.37 Amps at the full 25 Watt RF-power setting, so that is 42.29 Watts. (this is measured without the fan running) So that results in an efficiency of 59,11%, almost exactly the rated efficiency of the power transistor which was, as I mentioned earlier, 60%.
The chip that controls the display and remembers the settings is the 12C5A.
You can set the input audio volume electronically from 0 to 79 and the same for microphone input. Beware that you need a dynamic microphone and NOT an electret type microphone!! The microphone input is a mono channel (6.3mm jack plug needed). But using a microphone directly connected to the transmitter isn't very practical, unless you're transmitting at a public event or something. Otherwise I would use a microphone connected to an audio mixer and connect the mixer to the audio input of the T251. Audio input sensitivity is a bit less than on the CZE 7C. The input level is rated at ≤-15 dBV. The frequency response is: 50Hz to 15kHz (3dB). Distortion is 0.2% and the channel separation is 45dB (The channel separation of the average tuner/receiver is about 35dB so the transmitter is well above that.) I use a Philips MP3 player and I have its volume set all the way up and the same on the transmitter (volume to 79) and that get's the music exactly to the level of other stations. But it's better to use a pre-amplifier for the music, like a headphone amplifier or the line-out from an audio mixer to give yourself some leeway. I pre-record my programs and then run it through Adobe Audition which has a very good compressor plug-in by iZotope with a 'Broadcast' preset. That's what I use to render the radio show out and then transfer it to my MP3 player for broadcast. This works very well and the music quality is great. This unit has very tight bass and crisp highs. The sound of the 7 Watt CZE-7C is muddier in the lows and just doesn't reach the quality of the T251. Audio input is in the form of RCA connectors at the back of the unit. In the transmitter menu you can choose between stereo and mono transmission and it is my experience that the mono signal gives you a much bigger range.

More on the cooling fan:
The cooling fan kicks in automatically when the temperature reaches 39°C (102 F) and switches off again when the temperature drops to 35°C (95 F). It also starts running if you enter the menu to change settings. I guess this is done as a quick test to see if the fan is working. The fan produces no extra noise in the audio but it does produce a loud noise of itself so you don't want this transmitter near your microphone. Your studio must be in an other room than the transmitter or you'll hear the fan running in the background. You could of course replace the fan by a more silent one. It's not difficult to remove. The dimensions of the fan are: 40mm high x 40mm wide and 15mm thick. The hole spacing is 32mm. The fan get's 11,0 Volts when the transmitter is fed with 12,0 Volts. The fan's power cable has a 3 pin connector on it of which only the outer two are used. The middle pin is not connected. So if you get a replacement fan with a 2 pin connector you could just cut it off and solder the wires directly to the wires of the connector in the transmitter. The fan draws a current of 118 milliAmps at 11 Volts.
Fan Replacement Test:
I ordered some cheap silent fans from eBay and ran some tests. The first fan I tested was 40x40x10mm 12V 0.08 Amp. and this was totally not up to the job. It could not get the temperature down and kept running all the time. The second one I tested had a thickness of 15mm and a current draw of 0.1 Amp. It could get the temperature down enough to make it switch off again but only for the first 15 minutes. After that the heatsink was fully warmed up and too warm for this fan and it couldn't get the temperature down to where the fan would switch off again. So I reinstalled the old fan. So if you plan on replacing the fan you will need a type that is 40x40x15mm and with a minimum current draw of 150 milliAmps at 12 Volts. This is a picture of the fans I tested and which failed the test:

Here's a picture I took after the test, with the original fan again connected but not yet screwed into place. You can see the heat-sink arrangement and the 3 pin connector for the fan:

I couldn't actually find a good silent fan that was up to the job. 

Range vs Power:
With the T251 set to stereo, the full 25 Watts output power and a dipole 7 meters off the ground and in free air but not clearing the rooftops (see picture of antenna below) and an SWR of 1.1 to 1, I get a range of 10 kilometers (6¼ Miles) in a small-town setting without high-rise buildings. (Nothing higher than 5 stories to block the signal). That is a 10 kilometer range in which the audio is noise free received on a car radio driving away from the transmitter location (receive antenna on the rear of the car roof, so the ground-plane formed by the car roof is pointing away from the signal)! The range in which the signal can still be received is about 15 to 20 kilometers (9 to 12½ Miles) depending on terrain and obstructions. If you put the antenna high enough to clear the rooftops of most buildings near you, that can easily double! I also did a test in which I set the transmitter to an output power of 1 Watt and tried what range I had. It was unbelievable. I still had a range of about 5 kilometers (3 Miles). Sure the signal broke up much faster behind obstructions like buildings and bridges but still. 5 Kilometers is not bad. As I mentioned I set the T251 to 1 Watt, but measured on my Diamond SX600 SWR/Power meter an output power of 1.75 Watt. But seeing as it has to go through 7 meters of RG213 and 2 meters of RG58 I recon the Effective Radiated Power (E.R.P.) couldn't have been more than 500mWatts. That's impressive and shows once more that the most important thing about a transmitter set-up is the height and placement (and build quality) of the antenna!! Height is everything!

Here's a picture of my home built dipole antenna:

If you want to have a look at the Printed Circuit Boards inside the transmitter, then I refer you to the link below to my Flickr page where you'll find an album with some closeups of the PCB's:

Should you encounter any problems when operating this unit then there is an option you can try. If you switch the unit on with the switch at the back and at the same time keep the front dial button pressed in, the unit switches on into Self Test Mode. Here you can automatically check certain functions of the transmitter. Simply turn the dial to select the parameter you want to test and press to confirm. The unit then tests itself to see if it functions correctly. I personally have not tried this option yet because my unit functions perfectly fine and I am not comfortable doing this procedure when it's not necessary so I can't tell you exactly what it does when you select this option but just know that it is available should you need it.

This transmitter has one weird quirk. If you turn the front dial button whilst the unit is in sleep mode, with the button lit up red, or even if the unit is totally switched off, the temperature alarm can go off if you switch it on again! If this happens simply press the front dial button twice or switch off and on and you're good to go. A minor thing. Just don't play with the knob when it's switched off.

I've been getting some feedback that the output power is not really 25 Watt but more like 21 Watt. I myself measured this too on my SX600 SWR/Power meter but I thought it was my meter that was at fault; but I'm hearing from more people that they get the same measuring results. Now 4 Watt isn't a lot when you're already outputting 21 Watts so I shouldn't let it influence you if you're thinking about buying one of these units but I want this to be as honest a review as possible so I thought I would just mention it here.

On that note there is one more thing I must mention. A very small percentage of these units can fail and give a reading of zero output power and the SWR indicates 9.9 even though they go through quality control and are tested for a period of time before being sold. If that happens your only option is to contact the seller and try and get a new unit or a refund. The chances of this happening are extremely small but they are not 0%. Like anything you order from China there is a small gamble involved but usually the seller is very quick to replace the unit for you. It has happened to one person commenting below so you should be aware of this. But it is the first mention I received in the 2 years that this article is now online.

The unit I bought and tested performed very well. Certainly not a 'toy' as some people on YouTube like to say when they compare this to a $5000.- transmitter. That's like comparing apples to pears.
It simply is NOT a professional FM Broadcast transmitter but listening to it on your car radio no one is going to notice that. Stations that are 0.2 MHz or more away from your frequency won't be interfered with by your signal. And you don't need to be afraid of harmonic signals either. The Butterworth Low Pass Filter in the end stage suppresses that very effectively. But do be aware that around the direct vicinity of the transmitter location, stations can be surpressed by this unit. This is however only within a few hundred meters of the transmitters location and it's not always the case. It depends on the local situation of course and also on the quality of your antenna. Professional broadcast people curse this transmitter, and ones like it, because they say it creates interference on other frequencies. I can not see how it can do that with a lowpass filter behind the powertransistor but then again I am not an expert, just a consumer who reports his own experience with the unit for others to make of what they wish.
The sound quality is amazing. As I mentioned before, the cheaper CZE 7C's sound is phatter and is muddier in the bass region than the T251. This unit sounds very crisp and clear with deep tight bass and crisp highs. Really excellent I thought. Of course you must keep in mind that the perceived sound quality is first of all dependent on the quality of your receiver and audio system. Make sure to check whether your transmitter produces interference on other frequencies before you start using it. Some of these could have fake power transistors in them, although mine was fine.
If you want to buy a Chinese FM transmitter, I would get this one (or the cheaper 15 Watt CZE-15B which does have the BH1414K chip inside and is PC-controllable.)
I would stay away from the 60 to 80 dollar 15 Watt ones like the NIOrfNIO or the ST-15B. They are cheaper for a reason. Their quality is not terrible but comparable with the CZE 7C I mentioned above. But certainly stay away from the cheap FM kits. They use the KT0803L chip which is the worst FM chip you can get. Their signal is a mess, you'll black out the TVs in a full block radius. Spend a few extra dollars and buy better quality.

Here is a link to the USER MANUAL for the CZE T251 in PDF form:
Click here for User Manual PDF

Here is a link to the datasheet (PDF) of the RD30HVF1 Mosfet power transistor:

The comments below this article are proving to be a great resource. Reading them will give you an idea of other peoples experience with this unit and how it compares with other ones. Unfortunately I had to close the comment section for this article because it became too long and also because I've been out of the FM transmitter hobby for so long now that I don't feel qualified to answer any more questions about it. Sorry. But read the comments below and you'll probably find an answer to your question.

Disclaimer: The author does not accept any responsibility for actions and or alterations to equipment undertaken by anyone after reading this blog post. Anything you do, you carry the responsibility for.
Operating an FM broadcast transmitter without a licence is illegal in most countries, however, owning a transmitter but not using it as such is usually not illegal. Please check the laws of the country you live in to make sure you don't break any laws.