Waiting for spectrum analysser

The project is on hold until I can check the spectrum of the output.
I did use a scanner to check the harmonics and other spurious. It seems ok but this is a crude methode.
A real spectrum analyser is needed. This is a usefull piece of equipment so I am building one right now. See pa3emy SA project

TX mixer

The problem with the TX seems to be solved. It was not oscillating after all.

My tx mixer consists of a NE612 mixer. This device mixes the 10.7 MHz signal from the modulator with the 134 MHz signal from the PLL.
This signal is amplified by 2 MAR 6 amplifiers. Then the signal is boosted to 30 mWatt by a BFY90. Band filters are placed between the stages.

While peaking the band filters I found that the output jumps to maximum rapidly. I thought it was oscillation. Sometimes when power is turned off and on again the output is lower than the previous value. This is pretty weird.

The problem was in the second mar6 amplifier. It seems to behave like this when output is almost at maximum. So it looks like it was overloaded . I was able solve it by decreasing the resistor from the MAR6 to the 12V.

FM Modulator

Last night I worked on the 10.7 MHz FM modulator.
I use a super VXO to make a 10.7 FM modulated signal.
To get more deviation I put a varicap parallel to the existing varicap.
This works, it even brings the center frequency down 2 KHz. That solved another problem as the tx frequency was 2 KHz to high.

While I found another problem. It seems the driver stages osillates. I reduced the gain of this stage. I helps but it must be investigated further.

using repeaters

The TRX can use repeaters. I added a switch to automatically set the PLL frequency 600KHz lower when the PTT button is pressed. This works but I found out that I made a design error. The TX can transmit when the PLL is not in lock. For simplex this it is not a big problem as you usualy don't change frequency while transmitting.

For repeater use it is a problem though . As soon as the PTT switch is pressed the frequency changes and the TX is turned on. So It will transmit while tuning to the TX frequency. The TX/RX relay will probably prevent transmission because this relay switching is slow. To be safe I will delay switching on the power to the PA.

Another problem is that the repeater stations near to me need subaudio tones. I have no such encoder.

Power supply issue

It was necessary to place the power supply elsewhere.
Apparently the previous position was critical. On the new position the hum is gone.
Somehow the PLL module seems to be sensitive to the magnetic fields of the transformer.
Strange, as it is shielded. its 12 Volt input is decoupled by several capacitors.
Well anyway this problem is solved. fortunately enough space was available in the 19 inch box.

I also wired the ON/OFF switch.

The transceiver is now usable but I don't want to connect it to a antenna yet.
First I need to check the harmonics and other spurious emissions.

Next thing to fix is the 10.7 MHz FM modulator.

Some picures

Here are some pictures of the current state of the project.

Power supply, Power Amplifier,FM modulator, Microcontroller


PLL, Receiver, Driver


Front.

Power supply

Last night I was working on the power supply.
The 9.8 volt for the PA was still missing. Now it is in.
I tried to get a 78S10 for the power supply. This regulator can handle 2 Ampere.
Unfortunately It was unavailable so I used a 78S09 instead.
The transformer is rated at 1,26 Ampere. The radio draws a bit more, 1,4 Ampere.
So it is overloaded. Fortunately it seems to handle it well. It gets a bit warmer though. Beside that the output voltages drops because of this load.

When testing I found I had a lot of humm on the modulation. It turned out that the input of the 12V regulator is too low at minimum of the ripple.
Tonight I will increase the input capacitor to decrease the ripple.

S meter works

Now the S meter works.
I needed to make an extra buffer om the field strength output.
This was easy ,1 bc547 transistor and a ressistor were needed.
The S meter is not callibrated though. I don't care about that.
If it shows S9 correct than it is fine by me.

transmitter progress

The design of the Lowpass filter was found on the internet. It contains 2 inductors and 3 capacitors.
I have build 2 instances of this filter.

However it did not seem right. I doub that the component values are right. So did some simulation. I have used 5spice to. The filter responses looked strange. It has attenuation at 144 MHz. That can not be good.
So I changed the component values. Now it seems to work allright.Nevertheless I will have to check the transmitter using a spectrum analyser.

I get 7-8 watts out using a dummyload. Input power is 12 Watts. This look ok.
fortunately TX was stable whith an antenna connected.

I hope Power suppy can handle the 1.2 Amps that the PA needs. The transformer is rated at 1.2 ampere. I need 200-300 milli Amps for the other parts of the transceiver. So far I have used a separate powersupply for the PA.
The 19 Inch box has room for another powersupply. I have another transformer in the junkbox. so I will be able to solve the problem.

Receiver progress

It turned out that conneting a S meter did not reduce sensitivity at all. All it did was change the audio mute voltage of the squelch.
Unfortunately the fieldstrength output is unable to provide enough current for the S meter.
I need to make a buffer. Maybe I can change the buffer of the squelch circuit.

The problem with the spike noisse that causes unmute of the audio has been solved. A 100 nF capacitor between fieldstrength output and ground did the trick.

Power suppy humm

I have build a power suppy using the transformer I bought recently.
The powersupply is now build in the 19 inch box.
It supplies 12V and 5 V.

It worked but unfortunately a lot of 50 Hz humm was present on the PLL synthesizer.
Shielding and decoupling did not make a difference. I was able to reduce humm by rotating the transformer 90 degrees. This helped lot but I an not satisfied yet. I should be moved away from the PLL as possible. I need some mounting things for this.

Power amplifier.
I build the 8 watt power amplifier in. It worked well unit I connected it to the low pass filter.
It starts oscillating when the 9.8 volt is supplied.
Another problem is the TX/RX relay. When I use I get only 4 Watt out .