SIMPLE VHF FM TRANSCEIVER

Thanks to visit,

First time I knew about DRA818/SA 818 was from this site : http://www.kh-gps.de/dra.htm. Information on the website is provided with great detail, it also has example coding implementation using arduino. I grabbed the circuit with minor adjustment (my circuit) and design my own PCB (# KH001) using all 'trough hole' components for easy soldering with maintaining dimension of the board small enough (6 x 10 cm).

I want to build the radio to be simple to operate, minimize number of buttons on its front panel. It has only rotary encoder to dial the frequency (vfo mode), audio volume control and PTT button to transmit and functioning as a 'shift' button as well. Dialing encoder while PTT is pressed will offset TX frequency.

Assembling

KH001 transceiver PCB is designed for through-hole components, soldering process should be simple. The trick is to start with DRA818 module before putting anything else on the PCB, otherwise it may be difficult to make proper solder contact on its pad if your soldering iron is obstructed by other components. Using small solder tip is recommended for this purpose. Check for any unwanted solder bridge between pads or its metal shield. If all Ok, you can continue with small inductors L1, L2, L3, SMA connector, IC dip sockets, headers pin, and smaller form factor components (resistors, capacitors).

Prepare pre-programmed ATMEGA328p chip (recommended), or it can be programmed in circuit after assembling finished (6 pin ISP header provided on board), plug it in its socket carefully, please don’t spread her legs :)

Notes about circuit:

1. L1, L2, L3, C4, C5, C9, C11 and C13 is low pass filter network (value given for VHF version). It is optionally can be bypassed (by connecting pin 12 of DRA directly to the SMA connector). For example, you want to connect additional power amplifier then LPF need to be inserted at final output stage after PA.

2. I found that R6 (1K2) series with C18(10uF) between pin 1 and pin 8 of LM 386 are not really necessary. Gain produced by LM386 AF amplifier is enough to produce loud audio volume. I leave it up to you to add R6 and C18 for additional gain.

3. On the schematic diagram, we use DRA818 module, but it should works with SA818 as well (have not try yet). According to its data sheet, SA818 work with wider VCC range 3.3 - 5.5 volts vs 3.3 - 4.5 volts for DRA818.

LCD and Rotary Encoder wiring

LCD and Rotary Encoder connected to the 2x5 pin header, is shown on diagram below (notice that LCD display with I2C back pack is required here) : Of course, you can attached other display with I2C interface. OLED display will be nice ! . I use LCD just because I have it around in my spare box.

MIC wiring

Nothing sophisticated, i use less expensive Chinese HT microphone (Baofeng or else), it is sold under 5 USD in Jakarta ! I removed its original jack, and replace it with 4 pin CB type connector. Just remember, use only electret/condensor mic element since I've put DC bias resistor on PCB. Mic wiring diagram can be found on http://www.qsl.net/g4wpw/date.html

Pictures of fully assembled

Here what it is looked like, i built in an aluminium case . As a novice home brewer, I'm very happy with the project. Need more info or want to have the PCB ? contact me on my Google plus site. I may have some spare boards around.

TX spectrum : 2nd harmonic attenuation measurements

Thanks to Rich, VE3MKC. I use one of his great works PyDSA, a python script spectrum analyzer for Rigol 1054z oscilloscope to check tx spectrum (spourious)

I took three measurements at 134, 146.5 and 170 Mhz and found that attenuation at 2nd harmonic is at least 47 dB (it may need to be verified with a 'real' spectrum analyzer set).

Test setup picture :

Simply connect transceiver antenna out to CH1 Rigol (terminated with 50 ohm dummy load), rear usb port of the scope to my PyDSA PC.

Test Result :

@ 134 Mhz

@146.5 Mhz

@170 Mhz

Update : Ugh, You're too hot!

7805 is getting too warm quickly, it need a large heat sink. The radio will draw 1 amp current during transmit, with applying 12 volts input to 7805, roughly 7 watt will be dissipated by TO-220 package as heat. If you're curious, just check 7805 datasheet on how much 7 watt power dissipation can raise junction or packaging temperature.

I decided to add a small (and cheap) DC-DC converter module between 12 v DC input and the 7805, adjust its voltage to 8 volts (just above of minimum input voltage of 7805. Power dissipation is reduced significantly. Voila, it does the magic work !

Bye bye heat !

New update Dec 10th, 2017

I got a call from a friend recently, he was building the transceiver and experiencing a problem, it couldn't receive and transmit. LCD display was showing normal behavior, displaying frequency and its reading changes if he played with the rotary encoder, it seems SA818 module not responding/failed to work.

He double and triple checked soldering, wiring and all visually good (at the moment he called, he did not have test measurement such power meter/counter).

A bit hard to understand what was the problem until he told me he was using arduino UNO to burn his atmega 328. No, it should not be a problem as long as we are sure if it has correct clock setting. Chip was burned with arduino UNO's boot loader, meaning it expects 16 MHz clock, while it is actually using 8 Mhz crystal on the transceiver PCB. I suggested him to burn again the boot loader using 'arduino pro/mini' instead, which has 8 Mhz processor clock option. Wrong clock frequency makes UART has wrong baud rate, obviously it cant establish serial connection to the SA818.

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