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

[Zlatko1969]

First of all
thank you guys for replys
tech99 yes i read Tom points and i think that have a right.
1) The circuit shows a GND symbol and a wire labeled GND (+) RAIL. What is happening?
- yes GND or grounding point is (+) ,simply cannot be on (-) because this colpitts oscillator require grounnding
on (+), for example if i connect 330pF to MINUS, or 5nF then oscillator stop to work
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.
- yes it is different and that is why i ask in first place and i want to know if circuit #1 is a maybe better
solution.Also 330pF is connected to base of transistor not on GND like in my case.
3) L1 C7 resonate somewhere around 30-50MHz. Is that the frequency you need?
- this resonant circuit is a mistery to me and i cannot explain to myself what is a purpose of
33pF + 280nH which gives cca 52Mhz resonant frequency
No , of course that i don't need it . I just need to build ordinary FM mixer/oscillator
which should mix local oscillator signal with incoming FM sinal, heterodyne effect
as usual Fif = fosc - fin should give cca 10.7mhz
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"

 

[Baluncore]

When I simulate the redrawn circuit...
It takes time for the bias to settle with Vbase at about -2.0V. Once bias is settled, the LO amplitude varies in a sawtooth at the IF frequency. The IF and the LO are in series. It seems that high IF output voltage is robbing the LO of the voltage it needs to operate continuously.

I will need to think about how to fit those two signals into less than 2.5 volts.

 

[Baluncore]

I'm still thinking, meanwhile...
The following tuned circuits are connected by the non-linear transistor to make the LO/Mixer.
C3//L3 is resonant at the 10.7 MHz IF.
(C1+C4)//L2 is resonant at at the LO frequency.
L1//C7 should be resonant at the RF frequency, with low Q to accept all FM band.

It should not matter if C6 connects to the base or to AC ground. That is because this is a common base circuit, so the base is held at AC ground by C5. If the base is not firmly held to ground, then connection of C6 to the base would be an advantage as the RF tuned circuit would appear across the transistor's base emitter junction.

 

[Tom.G]

It seems that high IF output voltage is robbing the LO of the voltage it needs to operate continuously.

Try reducing the Q of the IF transformer; perhaps an R across L3, say 470R to 2k2. (Or just connect the secondary to the next stage. )

 

[Zlatko1969]

In some practical varsion of this oscillator i saw that is added ressistor
10k or 15k on primary side probably to dump unwanted oscillation .
so i will try that...

 

[Baluncore]

Well that is an interesting circuit.

The problem is with the LO and IF tuned circuits being in series. When first powered up, the LO starts quickly in a few microseconds. Then the amplitude of the IF increases gradually until it blocks the LO.

My model indicates that changing C2 from 5 pF to 10 or 15pF is sufficient to kill oscillation of the IF and stabilises the LO amplitude.

I changed L1 to 80 nH to be resonant in the RF range.

 

[Zlatko1969]

Ok
I will try to change bypass cap 5pF to 10 then i will see, also i will add
10k in paralele to primary coil of IFT.
Just to not forgot in some of versions i also see 100 ohm
collector resistor... probably also stabilises LO.

 

[Zlatko1969]

Guys
Is here someone who can give us better clear image of RFamp stage .
Image is too blur and i cannot see values of resistors and coils,capacitors

 

[tech99]

The oscillation of the circuit at IF, which blocks the LO each IF cycle, means that the circuit has become super regenerative. It will be very sensitive to signals but it will generate a lot of noise when no signal is present.

 

[Baluncore]

Is here someone who can give us better clear image of RFamp stage .

The circuit presented in post #1 was schematic and did not have values for all components.
First get the LO:Mixer operating, then redesign the RF front end. You need gain before the noisy mixer stage. Select an optimised RF transistor for the RF amplifier.

The aim with a receiver front-end is to use all available RF energy wisely. Where there is a Q problem you can adjust it simply by changing the reactance of the LC circuit components or by coupling more or less energy into the 1'st IF amp. The Q of the IF circuit will need to be about 10.7 MHz / 100 kHz = 107. Loading the IF input with a resistor wastes signal energy that could have been delivered to the IF strip.

As an example C3 = 47 pF brings the first IF transformer to resonance at 10.7 MHz. So the inductance of the transformer primary needs to be 4u7H. Then it has reactance XL = –Xc = 316 ohms. If you load that circuit with 1k0 you will bring the Q down to about 4, not the 100 needed.

For a Q of 100 the load should appear to be about 316 * 100 = 31.6 k. The 5:1 turns ratio transformer has an impedance ratio of 25:1. So the input impedance of the 1'st IF amplifier should appear to be 31k6 / 25 = 1k26 ohms. Otherwise juggle any of the values to get a happier solution.

 

[Zlatko1969]

Hi baluncore
i think that i understand your points.
I don't get it why i have problems because i use original
10.7Mhz IFT (orange) you may see in many FM receivers.
IF strip - amplifier work but it looks that I have big problems
between grounding.
In circuit shown here #1 is everything on MINUS rail so let say standard GND
when in my case is different GND of input RF amp and LO is on + RAIL of power supply

 

[Baluncore]

In circuit shown here #1 is everything on MINUS rail so let say standard GND when in my case is different GND of input RF amp and LO is on + RAIL of power supply

It is confusing, but I believe the AC coupling of the RF signal path with C8, and the IF with the transformer eliminates the supply problem.

I redrew your diagram using + and – power rails, with a separate AC ground symbol to clearly identify the DC bias circuit and the AC signal paths.

You are using one transistor for the LO and the mixer. Your one transistor is surrounded by RF, LO and IF, all of which must work together. There are always challenges with circuits that aim to minimise the number of active elements.

Reflex receivers were interesting because they used an amplifier for RF gain, then used the same amplifier again for audio output. https://en.wikipedia.org/wiki/Reflex_receiver

 

[Zlatko1969]

Yes i know that .
heh ..reflex receiver is easy to build and they work fine even with low selectivity
but this one is really confusing.
here is what i think that might work as RFpreamp