NE604 detector and micro controller

Fortunately I was able to get a NE604. I replace the AN7222 with the ne604. This device really works great. The dynamic range of the 38 MHZ receiver looks good. I measured 30 mV and .2 micro µV that would be 50 db dynamic range. My RF generator can not produce more than 30 mV so I van not verify that the the dynamic range is 60-70 db. It looks good though.

I am going to control the analyzer using a micro controller. This is quite a challenge. One of the problems is the generation of the sawtooth signal that sweeps the LO frequency. It could be done using a DAC. Then the micro controller knows the relationship between voltage and center frequency. It can display this frequency. So far so good. However this DAC needs a lot of bits. 12 bits are needed for 100 KHz steps (500/0.1). For 25 KHz 15 bits are needed. The output has to be very stable and without noise. After all 48mV drift means 1 MHz drift. It may be possible to use this concept but it is too difficult.

AN7222 Log detector not good enough

I found addition information about the log detector in the AN7222 AM/FM radio ic. The dynamic range just is not big enough for myb application. It can detect levels between 60 and 100 dbuV. That is just 40 db dynamic range. I had those AN7222 ic's in the junkbox so I used them. If I had no such ic's then I would have bought NE604 chips .This is a FM IF ic. I have used the NE604 in serveral projects. They work great.
So today I'll try to buy some. I hope they are still in stock.

38 MHz receiver

The tuner has an IF of 38 MHz so I need a receiver for 38 MHz for a fixed frequency.
The specs:
2 bandwidths 100 KHz and 25 KHz.
Log output.
Optional lineair output.
Dynamic range at least 70 db.

Design :

Building this should not be difficult. It is a straight forward superhetrodyne radio.
The 38 MHZ IF signal from the tuner is filtered by a bandpass filter. The Bandpass consists of three tuned circuits.
Then the signal is mixed in a ne612 with a 27 MHz signal out of a xtal oscillator.
This creates a 10.7 MHz signal which is filtered by a ceramic 10.7 IF filter.
An An7222 AM/FM receiver chip will be used as log detector.
The Ne 612 has two outputs. They can be used to make a balanced output signal. I use only one.
The other output is connected to another 10.7 MHz filter and log detector.
So I will have 2 bandwidths. It is experimental I have not seen this configuration before.

Last night I was working on it. The bandpass filter , mixer , one 10.7 Filter were build and tested.
One of the log detectors was build but I had no time to test it.

Start building spectrum analyser

I need a spectrum analyzer to test my home brew transceivers.
Unfortunately those things are expensive so I Will try to build one myself.

I may build more than one. The first one will be simple. just to check if my creations are performing according to the rules.
The other one will be more advanced. It will include a tracking generator and synthesizer control.
But that is for the future.

I found a TV tuner UDT-401 those things are hard to find. I was lucky to find one.
They can be used from 1 to 500 MHz. That is just what I need. It is enough to check the 2nd and third harmonic of a 144 MHz transmitter.

The tuner UDT-401
The UDT-401 has a few advantages over other tuners. These are.
1-500 MHz frequency range.
No range switching required
Flat response over the frequency range.
IF output at 38 MHZ.
1st LO frequency/256 available.
It has a few disadvantages though.
Hard to find.
2nd LO is free running. No control or output is availability.

Several spectrum analyzer designs exist that use this tuner. I have studied those designs. I can use some parts of those designs. However I want to use a micro controller to control it.
This part I have to design myself.