bitx40 tone beep ptt

A bitx40 has a transmitter power rating of some 7watts, just over the qrp of 5watts, but I thought it may still need a bit of help in crowded band's, during a pile-up situation. l have used the ssm2165 audio compressor chip, which in some ways similar to the Plessey VOGAD device. This was to ensure I did not distort the transmitter output, but I also found a website illustrating a DSB RF clipper circuit, perhaps for future modifications.

Below is the circuit implementation of the micro-controller ptt circuit using a picaxe 08m2 device. The application circuit is built on the 08m2 development board, as there is just enough room ( board space ) to do so. Added to the blog is a posting for the 08m2 sound command numbers to audio tone frequency, however I found by measuring the tone, I came to the various sound command values for each tone setting. For turning onto the TX mode a 1200Hz tone for 100ms is sent, and then return to Rx mode a 800Hz tone for 100ms is used. In both cases the ptt relay transistor holds open the ptt line for transmitter and then also holds open the ptt line for the completion of the Rx mode return beep. The 2N3904 NPN is used to mute the audio tones output, this is to prevent the low level digital noise from pin 5 entering the mic input amplifier. When the pips tones are not used, the mute function is enabled to switching out any digital noise from the sound output pin, this allowing a clean audio voice for transmission.

By installing the 08m2 development board, the program pins "C.5" and "C.0" are used for programming while in circuit of the bitx40 or any radio circuit. Device pin 7 "C.0" is also used for the mute of the pip tone audio using a 2N3904 NPN transistor. When programming the 08m2 device, be sure that the jumper selector is in the program position, then move jumper connector to the run mode to connect "C.0" pin 7 for program run mode.

Oh by the way, two modifications I have put to use are if one is using the official audio agc circuit for the bitx or ubit, then use an additional op-amp with a typical gain of some 10 times adjustable, inline between the audio tap on the drain circuit to the npn transistor amplifier input. This will amplify the capability of the audio agc design to both higher and lower Rx signal strengths. I used a TL082 op-amp fitted with a 50K ohm variable trim to adjust the signal gain. This gives also the second half of the TL082 op-amp to use as the signal meter amp for the bitx "Signal strength meter".

The second addition mod is the use of a 2n7000 mosfet across the agc capacitor, the mosfet activiated when the bitx or ubitx is in transmit mode. The problem found was that the agc capacitor got charged with a voltage during transmission, and when switched back to receive mode, the higher voltage on the agc capacitor as a result would cause a long delay of a discharge of the agc capacitor adding to the long disabling of the rx audio due to the agc voltage acquired during tx operation on the capacitor. By using a 2n7000 mosfet across the agc capacitor during tx mode, the voltage problem is overcome, and when switched back to rx mode, the rx audio is immediately present. The other thing that needs to done, is a capacitor to decouple the input to the 10K ohm resistor on the mosfet drain circuit, and then a capacitor to decouple the mosfet drain circuit output to the hot end of the volume control, then also use a capacitor to couple the volume control tap to the bitx AF amplifier. I tried serval op-amp interface circuits for the analogue meter, but in the end just gave up and tied the signal meter across the agc capacitor, trimmed the voltage multiplier trimmer for the analogue meter to max deflection needed.

Another addition is the low-pass filter to both the pip tone circuit and the ardiuno nano vfo board, using a 56ohm or so resistor and a 1000uF plus a 100nF capacitor to ground. This does two things, one the suppression of the vfo clicks, and second the removal of digital noise from the voltage supply line. This mod I got from YouTube video's regarding the bitx.

I found that the ssm2165 audio processor does not function in the similar manner as the Plessey VOCAD chip does. As a result, I found that the ssm2165 hung up the audio with a buzz tone, so I have now not used the speech processor to date, as I am not sure how to overcome the problem. The Plessey VOGAD uses a fixed gain amplifier to the speech audio then used an agc attenuator following the mic amplifier to then produce the controlled microphone level voice signal. The ssm2165 as far as I can determine, uses a variable gain amplifier to the agc voltage and this perhaps causes the tone buzz, even using 100pF capacitors placed in many places.

One other thing I found was the microphone to speaker feedback during switching from Rx to tx mode. The LM386 audio amplifier uses a 470uF capacitor on its voltage supply line, and during switch over from rx to tx mode, there is sufficient voltage delay decay time to allow the audio power amplifier to still operate. The mic amplifier output on the bitx board is connected to the audio amplifier pre-amp, this with a still operating LM386 for a short time during tx mode, the mic feedback buzz is caused. To overcome this problem, connect a second relay to the ptt relay transistor of the pip tone board to switch out the speaker during tx mode. The switched out speaker will prevent the mic to speaker buzz problem.

One other mod I added was to change the resistor R112 ( value of 4·7K ohms ) on the AF section pre-amp, this signal then feeding essentially the LM386 audio speaker amplifier. I found from a list of transistor pin voltages of the bitx40, that the collector voltage of Q16 is biased to 1·6Volts at a 12Volt dc supply line. I have re-biased the collector load resistance for a collector pin voltage of some 4Volts. I have been having some audio distortion on heavy sounding signals, and found that the new collector voltage of 4Volts on Q16 has had some improvement in audio reception with the receiver.

Mind you, the circuit application and code could be used for any home made or manufactured ham radio set, by adding it as an external microphone circuit.

For interest, the NASA over beep tone is 2450Hz for 200ms, while for the Apollo 13 movie, it was 2500Hz for 100ms ( http://www.repeater-builder.com/tech-info/courtesy-tones.html ).