Technical Specifications:. Suitable for the production of stereo wireless speakers, wireless microphone, wireless headphones, CD, MP3, DVD, PAD, notebook computer and other wireless audio adapter development and production. ROHM's new Japan has BH is one of the most simple and practical integrated circuits, which combines phase-locked loop circuit, stereo encoder circuit, transmitter circuit, as well as other additions. Pre-emphasis circuit, limiter circuit and low pass filter can significantly improve the sound quality. The total harmonic distortion up 0. BHF excellent frequency characteristics, it can achieve 40dB of isolation, transmitted sound quality is similar to local FM radio stations.

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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.

The company seems to do a lot of business in selling electronics parts, modules and kits. 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.

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 components are supplied loose in two zip-lock bags. This kind of component separation seems rather arbitrary. 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. The rear side of the page has the silkscreen, as well as a poorly reproduced version of the schematic.

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. A slight error seems to show the inductor near V3 labelled as L2, when it really should be L3. 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.

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. I began construction with the surface mount components, since they appeared to be one pressure point. The marking for the orientation of the BHF is a little subtle. 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. It feels like the footprint chosen was a little too wide. 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.

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. The underside looks pretty nice and well done, even though some of the connections came pretty close to the IC and other connections. 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. 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.

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.

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. The low band frequencies should have been 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. I hooked the antenna output without wire to my Rigol DSE oscilloscope with its probe set to 10x attenuation and properly compensated. Even though this oscilloscope officially only has a Mhz analog bandwidth, I figured a few Mhz above is no big issue.

In all, it seems that at The reduced voltage at the higher frequency might be a limitation of the bandwidth. It seems that there is an emission roughly near Mhz which is 20dB down, which might be harmonically related and undesirable.

At Into a 50 ohm antenna, this would be a transmit power of about 3. 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 dBuV output mV which would have only resulted in 0. Depending on how loud the input signal is, it is possible it seems to make the chip make very wide-emissions, possibly beyond the khz limit set by ACMA.

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. 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 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.

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. Thanks Lui, Your hard work preparing this page has been invaluable. I bought the kit quite a while ago and managed to put the instructions somewhere apart from the kit — mistake!

So, thanks to you! Bought this kit ebay nr. Thanks for the thorrow examination of this kit!!! But I forgot one question: is there any way to change that preset frequencies? I need another one, not only these presets. With 4-bit data input, only 16 possibilities are available, but two are reserved as disabling the PLL. Thus only 14 frequencies are available — There might be a small possibility of pulling the frequency slightly without modifying the crystal by changing the capacitance of the varactor diode circuit by adding capacitance, although it will be small in the pF range.

Just a thought. I hold the wire with tweezers, as they become very hot. By this way I dont need mechanic force, only a bit of scratching away of the rest of the burnt paint.

Works quite good and on this kit I did it as well with success. I have built 2 of these kits, and neither work. Although my experience with surface-mount soldering is limited, I think that the soldering is clean and the components properly mounted. I get no output on any of the frequencies, using an FM radio as a receiver and an audio signal generator as a source. Do you have any troubleshooting suggestions? Many thanks.

Rod P. Skip to content. 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. Construction Experience I began construction with the surface mount components, since they appeared to be one pressure point. Conclusion 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.

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