FM converter (mixer) -- I don't understand

[michael1978]

Sigh. Once 10.7 MHz was a high frequency. You needed shielded, tuned MF transformers and high frequency tubes. Now the mainstream CPUs use a clock frequency of at least 3500MHz! Thus, an entire FM receiver can be put inside an IC like this (http://circuitscheme.com/fm-receiver-with-tda7021t.html):
View attachment 233269

Hi, i buy long time ago, one chip tda7000 i think, but is easy to build, but you don't learn nothing, so now i already everything i know, how to build self, just a little bit, after i can use ic chip

 

[jim hardy]

That's a great way to learn, building from scratch.
ARRL Radio Amateur Handbook is a great reference. It will give you tips about layout . At high frequency stray capacitance can bring surprises.

 

[jim hardy]

I hazard that L4 - C16 functions to isolate the received signal from the Q3 oscillator feedback network. That is, it blocks the received signal from Q3-B and ensures that it drives Q3-E.

Cheers,
Tom

That's sort of along the lines i had thought.
It'd make more sense to me were it tuned to the station being received.
That's why i suggested set it midband.

 

[michael1978]

That's a great way to learn, building from scratch.
ARRL Radio Amateur Handbook is a great reference. It will give you tips about layout . At high frequency stray capacitance can bring surprises.

Thank you Jim i read a lot of times, but difficult to understand, but this book electronic communications for technecians is very goooood, i borow and i read.

 

[jim hardy]

Thank you Jim i read a lot of times, but difficult to understand, but this book electronic communications for technecians is very goooood, i borow and i read.

You're doing, actually building something.
You learn far more by doing than by reading about doing.

Spread those wings and fly, young man !

 

[michael1978]

You're doing, actually building something.
You learn far more by doing than by reading about doing.

Spread those wings and fly, young man !

Thanks Jim

 

[michael1978]

I hazard that L4 - C16 functions to isolate the received signal from the Q3 oscillator feedback network. That is, it blocks the received signal from Q3-B and ensures that it drives Q3-E.

Cheers,
Tom

Hello Tom
i try to simulate in ltspice, i tune L4 and C16 to 98mhz,? i get this result, is ok?

I hazard that L4 - C16 functions to isolate the received signal from the Q3 oscillator feedback network. That is, it blocks the received signal from Q3-B and ensures that it drives Q3-E.

Cheers,
Tom

HI Tom,
i try to experiment, and what do you think is this mixer good, and result good, output from collector
see

 

[Tom.G]

Does not look right. What are the parts values in the original circuit of C17B and of L1?
Also please simulate the circuit with source V1 having zero output and show the waveform and FFT traces again.

Cheers,
Tom

 

[michael1978]

Does not look right. What are the parts values in the original circuit of C17B and of L1?
Also please simulate the circuit with source V1 having zero output and show the waveform and FFT traces again.

Cheers,
Tom

Sorry tomi don't understand with V1 having zero output

i build the same result, wha do you think maybe wronk circuits? because also in the book they don't say nothing about L4 and C16

see picture, alsoi put the file if you need, i don't know anymore what to do, you have to rename cricut.txt circuit.asc

 

[michael1978]

I hazard that L4 - C16 functions to isolate the received signal from the Q3 oscillator feedback network. That is, it blocks the received signal from Q3-B and ensures that it drives Q3-E.

Cheers,
Tom

Hi Tom
i get this result without L4 AND C16

 

[Baluncore]

I think you need to draw the circuit in a sensible way.
C17a is used to tune the RF front end.
C17b is used with L1 to set the local oscillator frequency. C17b is labelled FM RF but it is actually the LO.
You should be able to see the Local Oscillator in the circuit as drawn here.

Varactor D3, (not shown), is used to trim the LO frequency, it moves the IF signal to center of FM detector slope.

 

[michael1978]

I think you need to draw the circuit in a sensible way.
C17a is used to tune the RF front end.
C17b is used with L1 to set the local oscillator frequency. C17b is labelled FM RF but it is actually the LO.
You should be able to see the Local Oscillator in the circuit as drawn here.

View attachment 233461
Varactor D3, (not shown), is used to trim the LO frequency, it moves the IF signal to center of FM detector slope.

First I apologize, i did want to make you tired, for the last time, you explain very good in details,
I apologize,
and thank you very muchhhhhhhh, you are a doctor

 

[michael1978]

I think you need to draw the circuit in a sensible way.
C17a is used to tune the RF front end.
C17b is used with L1 to set the local oscillator frequency. C17b is labelled FM RF but it is actually the LO.
You should be able to see the Local Oscillator in the circuit as drawn here.

View attachment 233461
Varactor D3, (not shown), is used to trim the LO frequency, it moves the IF signal to center of FM detector slope.

Hi Balounce one more time me, you forget bout C23, i done see, where you have to put in circuite between ? C5 is C23
Thank you.

 

[Baluncore]

It is difficult to simulate frequency mixers in spice. You can independently simulate the RF input impedance or the IF output network in the frequency domain, or oscillation of the LO in time, but not both time and frequency domains in the same run.

The way your example circuit was drawn effectively disguised the mixer in the LO where; Two signals are present on the non-linear emitter of Q3. One is the continuous oscillation of the LO, the other is the RF coupled into the emitter through C9. One of the cross-modulation products is selected by the tuned circuit in the collector, as the IF.

It is simple to understand once you look at the untangled circuit from the right direction.

Replace C5, 100 pF with C23, 300pF. I think I was thrown by having to guess the missing value for C14. What was the value of C14 before you put the solid black line of misunderstanding through it ?

 

[michael1978]

GoodMoring Sir, ithe value of C14 IS 5nf, and this circuit i take from the book, he explain almost everything but not all, i replace C5 with C23 300pFTHANK YOU...…………….

 

[michael1978]

It is difficult to simulate frequency mixers in spice. You can independently simulate the RF input impedance or the IF output network in the frequency domain, or oscillation of the LO in time, but not both time and frequency domains in the same run.

The way your example circuit was drawn effectively disguised the mixer in the LO where; Two signals are present on the non-linear emitter of Q3. One is the continuous oscillation of the LO, the other is the RF coupled into the emitter through C9. One of the cross-modulation products is selected by the tuned circuit in the collector, as the IF.

It is simple to understand once you look at the untangled circuit from the right direction.

Replace C5, 100 pF with C23, 300pF. I think I was thrown by having to guess the missing value for C14. What was the value of C14 before you put the solid black line of misunderstanding through it ?

Hi Baluncore my last question for this circuit, L4 and C16 are tuned to 92MegHz?

 

[Baluncore]

Hi Baluncore my last question for this circuit, L4 and C16 are tuned to 92MegHz?

That was just a wild initial guess for L4. My later version LTspice model oscillates and mixes OK, producing an IF output, but it has not been optimised or tuned to any particular IF or FM allocation. There is no point talking circuit values without frequency band and IF specifications.

I don't think we can arbitrarily assign a frequency of 92MHz to L4//C16 which would probably be in the middle of an FM RF band. The value of L4//C16 will also affect the LO resonant frequency. Why was a fixed value for L4 not specified? What is the purpose of L4//C16. What should the frequency and Q of L4//C16 be?

The RF frequency range is country dependent. It will be a VHF block, somewhere in the range between 66MHz and 108MHz.
The IF centre frequency is not specified, I would guess at 10.7MHz, but it could be as low as 5MHz or as high as 22MHz.
The LO range would be offset, probably above, but possibly below, the RF band by the IF centre frequency.

For 10.7MHz with C16=30pF, L4 would need to be 7.4uH.
For 30.MHz with C16=30pF, L4 would need to be about 1uH.
For 92.0MHz with C16=30pF, L4 would need to be 100nH.

 

[michael1978]

Thank you very much Baluncore, have a nice dayyyyyy

 

[michael1978]

That was just a wild initial guess for L4. My later version LTspice model oscillates and mixes OK, producing an IF output, but it has not been optimised or tuned to any particular IF or FM allocation. There is no point talking circuit values without frequency band and IF specifications.

I don't think we can arbitrarily assign a frequency of 92MHz to L4//C16 which would probably be in the middle of an FM RF band. The value of L4//C16 will also affect the LO resonant frequency. Why was a fixed value for L4 not specified? What is the purpose of L4//C16. What should the frequency and Q of L4//C16 be?

The RF frequency range is country dependent. It will be a VHF block, somewhere in the range between 66MHz and 108MHz.
The IF centre frequency is not specified, I would guess at 10.7MHz, but it could be as low as 5MHz or as high as 22MHz.
The LO range would be offset, probably above, but possibly below, the RF band by the IF centre frequency.

For 10.7MHz with C16=30pF, L4 would need to be 7.4uH.
For 30.MHz with C16=30pF, L4 would need to be about 1uH.
For 92.0MHz with C16=30pF, L4 would need to be 100nH.

THANKYOU A LOTTT...HAVE A NICE DAY...

 

[Zlatko1969]

Hi
I see that this topic is from last year.
But i am interested is this oscillator tested in practice and how work?
Also is there a way to force ocillator to work on 3V?
thanks

 

[Baluncore]

Welcome to PF.
What is your application for an oscillator? What frequency?

Why do you need this particular oscillator?

I see no reason why it should not work on a 3V supply, but component values will need to be changed. Do you have the free LTspice program?
https://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html

 

[Zlatko1969]

Hi
It is clear for what frequency is
It is for standard FM 88-108Mhz superhet receiver.
I am intersted how this oscillator perform in FM receiver because i build vary similar
which work on 3V battery.
In my circuit i used S9018 transistor in common base mode with (+) rail GND.

No i don't use LTspice than CircuitMaker free student version.

Also i am asking because in my version transistor 2N3904 not work.

 

[tech99]

It seems likely to me that the 2N3904 does not have sufficient gain at 3V with a small collector current. The transition frequency is quoted as typically 300 MHz, which is a bit low already.

 

[Baluncore]

Also i am asking because in my version transistor 2N3904 not work.

What circuit do you use now?
Please drag and drop your circuit diagram.jpg onto your next post.
We will look at your circuit to find the problem.

 

[Zlatko1969]

Hi
2N3904 is a good transistor and work well in circuits like
regen or superregen receiver but you cannot found it in any
comercial oscillator/ mixer.
I have problem in current circuit even with S9018 not with transistor
than with transformer coupling capacitor which simply to much dump
oscillation( typical rush "hiss" is reduced too much.
Ok here is the schematic with grounded (+) rail.

 

[Zlatko1969]

So that is why I asking
Is it better to have grounded MINUS rail?

 

[Zlatko1969]

I forget to tell you L1 is a coil 19 Turns/dia 3mm - cca 280nH
what is purpose of this coil in combination with C 7 33pF ?
which resonantt freq is 52 Mhz ?
that is a really strange...

 

[tech99]

Hi
I have problem in current circuit even with S9018 not with transistor
than with transformer coupling capacitor which simply to much dump
oscillation( typical rush "hiss" is reduced too much.

Are you saying your circuit is super regenerating i.e. it is loudly hissing?

 

[Zlatko1969]

No ...it is typical "hiss " like in any comercial FM receiver when is not on station.

 

[tech99]

Are you saying the feedback capacitor is too small so that oscillation stops?

 

[Baluncore]

Ok here is the schematic with grounded (+) rail.

I cannot see any reason why your circuit should not work. The modeled local oscillator, LO, starts quickly and oscillates. The 2N3904 was used in the earlier simulation because it is a commodity model in LTspice, there will be better choices for low power RF circuits.

In your circuit, the emitter current is set by R3 =1k0. The base current is set by R1 = 150k. The ratio of those two resistors sets the share of the 3V supply, and so the operating point. But that ratio
is also sensitive to the current gain of the transistor.

RF = 88 to 108 Mhz superhet with IF = 10.7 MHz; ∴High side LO = 98.7 to 118.7 MHz.
Can you tell if your LO is oscillating? You might set your osc:mixer to the low frequency end of the band, then listen to your LO leakage mute a commercial FM receiver.

 

[Zlatko1969]

-Can you tell if your LO is oscillating?

Yes it oscillating because there is a hiss and i can tune few strongest station
but is unstable ,so that is why I ask is first version of this topic maybe better and more stable.

 

[Tom.G]

Yes it oscillating because there is a hiss and i can tune few strongest station
but is unstable

That describes a stage somewhere is doing un-controlled oscillation.

Ok here is the schematic with grounded (+) rail.

A few problems I see:
1) The circuit shows a GND symbol and a wire labeled GND (+) RAIL. What is happening?
2) The Base wiring in this circuit is a lot different than in post #1. For instance C6 here connects to GND (+) RAIL but the original circuit shows that capacitor connected to the Base of Q1.
3) L1 C7 resonate somewhere around 30-50MHz. Is that the frequency you need?

That miswire of C6 is probably a cause of the "unstable" you describe.
L1 C3 frequency could be the problem of "can tune few strongest station".

Cheers,
Tom

 

[tech99]

That describes a stage somewhere is doing un-controlled oscillation.

Zlatko: When you say "unstable", do you mean that the frequency varies too much, or do you notice unwanted oscillation of some type? Also, please notice the circuit questions from Tom.

 

[Baluncore]

I redrew the circuit to check the bias current circuit and the AC ground.