While I was browsing the listings of electronics kits on eBay, I came across this particular stereo FM transmitter kit but I didn’t think much of it. In fact, given the chance to buy it, I passed over it twice. It was only when I started running out of kits to buy and build that I decided to buy it.
This kit is a little unlike the other kits I had attempted to date, namely that this kit has a brand – YYDZW. It doesn’t roll off the tongue, because it’s merely the initials of the Chinese company name. The company seems to do a lot of business in selling electronics parts, modules and kits. In all, this kit costs just AU$10.99 including postage, which makes it another low-cost way to keep yourself entertained on an afternoon. One particular thing I liked was the use of the Rohm BH1417F integrated circuit, which reminds me of one of the earliest kits I built (the Silicon Chip Mini-mitter) which used the Rohm BA1404 IC for much the same purpose.
Unboxing the Kit
We know the drill by now – the kits just come as a mash of components inside a plastic bag, and this one is not much different. On the plus side, the bag is resealable, so I suppose you could use it to hold the kit once it’s finished.
Inside, the main star is a single sided fibreglass type PCB with silkscreen. Capacitor orientations are clearly marked, along with microphone polarity. Values for components are omitted, so reference to the printed material is required. The unit does make extremely good utilization of the PCB space, with a fairly dense component layout which could make it tricky for people with larger fingers. As this kit is branded, the branding is printed along the edges including two website addresses, a contact phone number and QQ account.
The rear of the board has minor silkscreening, but is mostly covered in green solder-mask and bright tin-plated pads. The PCB feels like it’s of good quality and looks like it would be easy to solder, maybe with the exception of having some silkscreen “mess” along the left side on the top side of the board.
The components are supplied loose in two zip-lock bags. One bag contains the ceramic capacitors and surface mount components, while the other contains “the rest”. This kind of component separation seems rather arbitrary. It’s also rather ordinary (but common) that ESD sensitive parts are thrown into such non-ESD safe bags, although at least in this case, those parts are still encapsulated in the tape. Two of the SMD components are extremely small, so having a good set of tweezers, a steady hand and good eyesight is a must.
Along with the components, we are supplied a double-sided single A4 sheet of information, all of the text being in Chinese of course. I suppose that just adds to the challenge. From what I can tell, it claims to be a “three star” medium difficulty kit with BH1417F (Stereo FM Transmitter IC), KV1471 (Varicap Diode) and 2SD2142 (NPN Darlington Transistor) being the surface mount components involved.
The rear side of the page has the silkscreen, as well as a poorly reproduced version of the schematic. Some of the print has rubbed off from the folding and transit, but it seems that from the left, there are two “blocks” which represent the microphone and pre-amp. The DIP switch positions 5 and 6 control power to the two microphones, and if set to low, turn off the microphone input for the audio jack instead. The BH1417F performs most of the magic with the DIP switch positions 1 to 4 setting D3 to D0 in “inverse” – i.e. where set to high, it sets the input to logic zero (low). An 7805 performs voltage regulation to produce 5v from the input socket, however, it seems that the input power is used to drive V3 (9018) as a single transistor amplifier for the modulated FM output to the antenna, with L1 and C39/C40 probably acting as a filter so that the FM signal doesn’t propagate into the power supply and use those leads as an antenna. The varicap and Darlington pair are both part of the PLL network which allows the chip to “electrically” tune the frequency.
A slight error seems to show the inductor near V3 labelled as L2, when it really should be L3. There might be a few other errors as well, although I didn’t take the time to check it out. If you can read Chinese, there is a good amount of text on the front side (not reproduced here) which gives some basic overview of the circuit operation principles.
The eBay listing I bought this unit from seems to have a picture of the kit, but it’s obviously incomplete without several of the components on the board. It recommends doing alignments with 9v batteries to ensure best sound quality (as power supply noise will impact the performance). Unfortunately, it seems that this kit is not provided with any 2.1mm DC jack or battery snap – so user will have to provide their own compatible positive-tip power supply, plug and or battery snap to test and or utilize the kit.
I began construction with the surface mount components, since they appeared to be one pressure point. The whole idea being, if you can’t get the SMD components right, you really don’t have a kit. The marking for the orientation of the BH1417F is a little subtle. The orientation of the Darlington transistor is self-obvious, although due to its small size, it can be quite a challenge to solder and maintain its orientation. The orientation of the varicap diode required some deduction based on the schematics. I believe it faces with the negative band pointing right as in the picture below. The use of such small SMD components makes this kit somewhat beginner unfriendly – as well as the close spacing of pads which can provoke higher incidences of solder bridges. Since that bag of components was already opened, I decided to also go ahead and mount the ceramic capacitors.
The underside looked like this. I think I did a pretty decent job with the soldering so far. Another observation was that many pads connected to the ground plane has no thermal isolation, making soldering a little trickier – you need to leave the iron on those pads for longer as they draw away the heat more rapidly, and thus don’t heat up as quickly as island pads. The pads for the BH1417F were also very much quite far apart, so when the device was placed on the pads, it really only had its “tips” touching the pads. It feels like the footprint chosen was a little too wide.
While constructing, it was also noted on the top side that the silkscreen orientation and number order isn’t always logical, which requires a bit of time “hunting” for the right location.
After two hours of fitting components, soldering and clipping the leads, the kit was completed. Specific attention needs to be paid to the orientation of ICs, transistors, polarized capacitors and microphones. For those who haven’t dealt with electret microphones before, the wire connected to the shell is the negative pin. Because the microphones are only supported by their connecting wires and are otherwise free-standing, care needs to be taken not to fracture the wires. Hot glue should probably be used if you have it.
While constructing the kit, the inductors proved to be a challenge. The wire comes pre-formed into a loop with one too many turns, so that needed to be undone. Then the ends need to have their enamel scraped off and then tinned. Unfortunately, it seems the enamel was quite hard, so a bit of pressure was necessary. The blade snagged the wire of L2, resulting in it turning into a kinked mess. I reformed it as best as I could around a small screwdriver shaft, but it looks pretty shambolic. At least L3 didn’t get damaged the same way.
The underside looks pretty nice and well done, even though some of the connections came pretty close to the IC and other connections. The number of components supplied were exactly as required – so no disappointments here, although if you do end up losing or damaging any, you’ll have to go hunting for a spare.
Tuning and Testing
I cut a piece of random length wire and soldered it to the ANT1 terminal. Because the kit was advertised as transmitting on a number of discrete DIP-switch selected frequencies, utilizing PLL techniques for stability, I ambitiously set the jumper switches to something sensible and powered the unit up, expecting to see a signal at the given frequency.
Unfortunately, things are not as simple as this. Instead of seeing a signal at the frequency I had wanted, I found a signal at a lower frequency which wavered and had some whining noises to it. This appeared to be a symptom of PLL unlock, so I put my attention towards the PLL area.
The key to tuning is to massage L2 until the inductance is a value which the varicap can compensate for, thus bringing the PLL into lock. To do this, you can separate/squeeze the turns on L2 while watching the signal on an RTL-SDR/spectrum analyzer. At the point you modify the inductance, the frequency will “jump” way off, but will slowly re-converge to where the transmission should be.
After a little bit of massaging L2, you should be able to find an arrangement where the unit converges and holds to the listed frequencies. Changing the DIP switches should result in the frequency sliding over to and locking at the expected frequency.
In my case, as my L2 was “damaged” during an attempt to strip the enamel ends, the re-wound L2 was a bit lumpy and mis-shapen. I assume that it is, because of this, I was not able to achieve proper PLL locking for the low band frequencies, only for the upper.
DIP Switches 1234 LLLL [LAST FREQ KEPT] LLLH 107.9 Stable LLHL 107.7 Stable LLHH 107.5 Stable LHLL 107.3 Stable LHLH 107.1 Stable LHHL 106.9 Stable LHHH 106.7 Stable HLLL [LAST FREQ KEPT] HLLH 94.9 Unstable HLHL As Above HLHH As Above HHLL As Above HHLH As Above HHHL As Above HHHH As Above DIP Switch 5 and 6 control power to the microphone preamps.
The low band frequencies should have been 88.9, 88.7, 88.5, 88.3, 88.1, 87.9 and 87.7Mhz respectively. The loss of low-band frequencies was not particularly problematic, as those frequencies tend to be interfered with in my experience.
You can adjust the sensitivity of each microphone channel by playing with the potentiometers. Otherwise, you can disable the microphones themselves by turning DIP switches 5 and 6 to the off position, which cuts the power to the preamps. Hooking up an external source using a 3.5mm to 3.5mm stereo cable allows you to transmit from analog sources.
I hooked the antenna output (without wire) to my Rigol DS1102E oscilloscope with its probe set to 10x attenuation and properly compensated. Even though this oscilloscope officially only has a 100Mhz analog bandwidth, I figured a few Mhz above is no big issue.
In all, it seems that at 106.7Mhz, it registered 377mV RMS, and at 107.9Mhz, it registered 347mV RMS. The reduced voltage at the higher frequency might be a limitation of the bandwidth. It seems that there is an emission roughly near 310Mhz which is 20dB down, which might be harmonically related and undesirable.
At 95.15Mhz (low band, unlocked), the output voltage was 400mV RMS. Into a 50 ohm antenna, this would be a transmit power of about 3.2mW. This seems to indicate the final stage single-transistor amplifier is doing some good (about a 4x gain), since the Rohm chip claims to have a 100dBuV output (100mV) which would have only resulted in 0.2mW.
This actually is quite a lot more than the 10uW that ACMA allows in Australia, although at such a low output power, it’s unlikely anyone will be after you. It does seem to be a fair amount more power than the USB FM transmitter I tested earlier by other means.
Depending on how loud the input signal is, it is possible (it seems) to make the chip make very wide-emissions, possibly beyond the 180khz limit set by ACMA. That being said, even when at “silence” with the microphone input, it seems there are quite a few spurs and hiss/whines which may be partly switching power-supply related (as I used an external hard drive power supply for testing). The audio output quality as demodulated by SDR# with an RTL-SDR dongle was not particularly noteworthy, with quite a bit of hiss and noise, although it does beat all of the old single-transistor FM bugs hands-down in this price segment. As my RSA306 demo for the FM audio analysis had expired, I didn’t do any extra testing of the audio output quality – it really isn’t that special.
So if you think of it as a “bug” replacement, you get stereo transmission, the possibility to use your own source, the stability of crystal reference PLL and more output power at roughly the same price. Just don’t expect hi-fi quality.
On the positive side, the kit had all the necessary components in the right quantities, and used a quality single-sided fibreglass board with tinned pads, solder mask and silkscreening for easier construction. The price is also extremely acceptable, considering that this kit does do stereo transmission – for the same price, you’re likely only to be able to purchase a monaural FM bug kit with no PLL resulting in wild frequency drifts.
The downsides include the use of a number of surface mount chips with the smaller ones being quite challenging for beginners. The lack of component value reference on the silkscreening required reference to the printed material which was poorly reproduced, and the direction of the diode was not immediately clear. The order in which the components were numbered on the silkscreen did not appear to be logical in some cases, resulting in hunting around the board for the right point to place components. The enamelled copper wire proved hard to strip with a knife and difficult to solder, and its value appears to be critical to achieving successful high and low band operation (as a result, I was only able to achieve high-band operation). The lack of English based documentation probably makes this an additional challenge, although not insurmountable.
On the whole, the performance seemed somewhat consistent with my earlier mini-mitter efforts – its transmit power was sufficient for short-to-medium range household transmission, but the audio did have some level of background noise and the pilot tone may have been a little strong and/or causing intermodulation. Overmodulation was possible given a high enough signal level. However, considering the price and nature of the kit, it seems like you do get good value for money.