FM modulator block diagram

This is the block diagram of the FM modulator.
The signal from the microphone is amplified. Then the signal is clipped and filtered.
After that it is used to modulate a super VXO at 10.7 MHz. The signal out of the super VXO is buffered and filtered.

It took some effort to get enough deviation. Getting deviation was not that difficult. It was difficult to get enough deviation and a accurate carrier frequency. I managed to achieve this by used 2 varicaps parallel. The resulting FM sounds alright on a HF receiver tuned at 10.7 MHz.

Abandoned concept:

I tried a different modulator first. I made a PLL at 10.7 MHz using a 74??4046. This chip contains a VCO and a phase detector. FM was made by modulating the loop. It worded but it sounded awful. I found out that the PLL locked when the loop was open. It had a small catch area. Due to some internal coupling I guess.So I used a second a 74??4046 as VCO. It worked. The FM sounded excellent. However I could not make enough deviation without adding a lot of distortion. I don't know why. I think the concept is good the implementation must be wrong. Maybe the phase detector switches to frequency detector mode for high deviations. I stopped working at it as it would take too much time to figure it out. If I had succeeded I would have had a HiFi modulator. This would be overkill for a communication transceiver anyway . So I built a modulator using a VXO.

transmitter block diagram

This diagram shows the transmitter section.
It is straight forward. The mixer mixes the signal from the synthesizer with the 10.7 MHz signal from the FM modulator. The result is a 144 MHz signal. This signal is filtered and amplified. I have used several band filters to remove unwanted mix products. Finally the signal is amplified by a power module to 5 Watt. I have cascaded 2 low pass filters to get rid of the harmonics.

Tests with a spectrum analyzer have to be performed to check if the filtering is OK.

block diagram receiver

The receiver design is straightforward. See picture.
Only the squelch circuit is special. The NE604 has a mute input. I made a comparator with discrete components. The comparator checks the level output with the squelch threshold. If the signal is below the threshold the mute is activated.

This receiver had been build a few years ago. It did work but I was not satisfied. It had no squelch, the demodulated signal was weak, it had no preamplifier. The mixer was a BF960. I improved all those issues. Now it works fine.

The preamplifier is a mosfet with a tuned circuit at the gate and one at the drain. It has an issue. When both the input filter as the output filter are peaked at maximum the preamplifier oscillates. So it t needs a small misalignment.
The input is shielded form the output but that does not solve the problem completely.

block diagram PLL

This picture shows the PLL synthesizer. The concept is straight forward.
The VCO signal is mixed with the 132,3 MHz signal from the tripler.
This produces a signal between 1 and 3 MHz. The 4049 programmable divider can handle these frequencies. Phase detection is done by a 4046 phase detector. The reference signal 12.5 KHz comes from a 4060 (f/64) followed by a 74194 divider (f/5).

I have build this synthesizer years ago. The plan was to use it in a transceiver once. So it uses some older ic's. Back then I had some recent experience with these components so used them. If I had to do it again I would use modern components.
I improved several parts of the PLL. For example the step size. It used to be 25 KHz steps.

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 .

First Contact

I have had the first contact with the radio. I just solved the problem with the pll jitter.
It turned out to be bad connections and maybe a instable capacitor in the vco.
Then a fellow ham responded when I was testing.

Only the tx driver (30mWatt) was working and no antenna was connected.
The other station was only a few hundred meter away from my QTH.
Connecting a piece of wire to the driver boosted my signal to almost S9.
Great, the first contact has been made before construction has finished.

I bought a transformer for the power supply. It fits perfectly. However I may need a bit more current than this transformer can deliver. I will add add an extra power supply if necessary.

Next problems on the 'to be solved list':
-Increase high tones in the audio.
-Solve squelch problem. Spike noise is audible when squelch is closed.
-Solve s meter problem. When connecting a meter to the sensitivity of the receiver decreases.
I would like to show some pictures. Unfortunately my camera broke down.

pll jitter

This weekend I did some wiring.

The powersupply is still not in. I have no transformer that fits the box. It should be smaller than 4.5 cm.

TX driver and the RX work. However the PLL has more jitter than it had on the breadboard.
I have checked it using a scanner It makes craking noises. The 10.7 MHz modulator sounds Ok.
Tonight I'll have a closer look at it.

It takes more time than expected.

building the tin boxes in the 19inch box

Last night I built the tin boxes in the 19 inch box.
Then it was time to wire them. This took longer than expected. In fact it is still not finished.
To night I will try to finish it.

I build in some relays for switching between receive and transmit.

I had to solve a short circuit. It was quite hard to find. One of the screws made contact with a tin can mounted on the other side.

Next problem will be the power supply. Three voltages are needed 13.8V , 5V and 9.8 Volt.
The 9.8 Volt is for the PA module. This is non standard I suppose The PA module was designed to be fed from Ni cad batteries. I will have to build this power supply myself.

Holes in the box

19 April
I finnaly finished making holes in the 19 inch box. Now I can start builing the tin cans in the box.

Start of Blog

In this Blog I will describe the design and construction of a 144 MHz transceiver.
The goal is to build a FM tranceiver from scratch. The frequency range must be: 144 MHz to 146 MHz in steps of 12.5 KHz. It should be possible to use repeaters.
The frequency must be controlled by a PLL. A microcontroller will control the PLL.
Output power will be at least 5 Watt.


A prototype exists on a breadboard . Basically it works but it still needs a lot of work.

This picture shows the receiver (left), the modulator (middle) ,the PA (right) and the lowpass filter.

The picture shows the microcontroller (left), the PLL unit (middle) and the receiver (right).

These units including a power suppy will be build into a 19 Inch Box. I am not sure if the power supply will fit.