Some like to take the trouble to explain circuit?

[flat5]

Item: https://www.banggood.com/DIY-Radio-...ter-Kit-Receiver-7_023-7_026MHz-p-973111.html
Transmitter is ok. Receiver sucks. Some type of regen. Lots of hiss but sensitivity is terrible. Adjustment pot does not seem to do anything or very little. Coupling a signal generator to the antenna gives just enough received signal to know that it is receiving. Almost nothing. No quieting.

I'm trying to understand the receiver. Having lots of trouble. For one, the antenna coupling is confusing me.
No point in me rambling further.
If someone would explain to me how this receiver works I'd really appreciate it. I don't think I'd find this circuit on-line and explained.

 

[JimB]

This is a fairly well known circuit in amateur radio circles, it is known as "The Pixie"
Googling "Pixie 40m" gives lots of hits.

The basic receive circuit is known as a "Direct Conversion" or DC.
Basic principle is that of a superhet, but the intermediate frequency is zero. So the RF is converted directly to audio.

Yes the sensitivity will be poor.

When you say "No quieting", please be aware that the term quieting is really only applicable to FM receivers where the discriminator output is noise in the absence of a signal, not to detectors of the various AM modes.

JimB

 

[cowboybob]

Did you try (as was suggested) to enclose the transceiver in a metal box (shield for ambient noise). That might reduce/eliminate the "hiss".

You might also carefully check all your soldering points.

 

[flat5]

There will be hiss because it's a regen. Or so thinks I.
It is not in a box. I did leave the bnc off the board for now and soldered the 51 ohm resistor in it's place.
So the antenna input is terminated by 51 ohms and a clip lead is on the high side of the load and very close to the rf generator probe.

I have checked the wiring which is not such a problem on this board. Pads are 'far' apart mostly.

 

[JimB]

There will be hiss because it's a regen. Or so thinks I.

You thinks wrong.
As I said earlier it is a direct conversion.

JimB

 

[flat5]

Sorry Jim. I responded to the post following your's before I saw your's.
The doorbell sounded and a friend brought a toy robot board over for me to troubleshoot.
Just finished and am now responding to your post.
Meaning I did not ignore you, just got (pleasantly) sidetracked.

I still am confused about the antenna coupling for the receiver.
It makes perfect sense for the transmitter.
Now I go Google "The Pixie".
Thank you for your help.
You too, cowboybob (who looks more like a fisherman).

 

[JimB]

The "antenna coupling" is simply a low pass filter.
The LPF is necessary to reduce the harmonic output of the transmitter, and limits the receivers sensitivity at harmonics of the crystal frequency.

JimB

 

[dr pepper]

Noise may well be generated in the front end transistors, esp if this is a high gain circuit, do you really need wideband response, if not you could put a filtering cap or simple network across the audio amp and loose most of the hiss.

 

[flat5]

Jim, like I said, the PI network (am I correct about that) is normal for a transmitter.
Coupling into the collector on receive is what is troubling me. How does that make sense?

Quieting may be the wrong term, but in a regen (ok it's not a regen) there is a lowering of noise when a signal is received much like quieting but not as much as in a strong fm signal, if I remember correctly. Regens scream too.

dr pepper, removing the extra gain cap on the 386 is said to lower noise quite a bit.
I could start there.

A direct conversion receiver should be very quiet except maybe have hum.
The noise level is so high now that any hum will not be noticed.

I'm thinking perhaps a wide band oscillation is happening.
I should scope the transistors and the input/output of the 386 and see what I can learn...or at least see

The Pixie. Have not studied the pdf files yet.
**broken link removed**
**broken link removed**
**broken link removed**

 

[JimB]

Coupling into the collector on receive is what is troubling me. How does that make sense?

It does not!
I have not studied the circuit in detail, but my first thought was that it was using the base-collector junction as a diode mixer.

JimB

 

[flat5]

Broadly I can see an oscillator and buffer on transmit.
The circuit is tricky so I don't know what all the parts are doing.
Do r6, c6 ,w1 have a function on transmit?

On receive is the signal path q2 collector-base, c4, c3, y1, r6, w1, c10...

I've never seen an amplifier circuit where the bass and emitter are at the same dc potential.
I have not worked closely with class c rf amps though.

I don't know what w1 is for or why it is adjustable.

 

[jjw]

...

I don't know what w1 is for or why it is adjustable.

I think w1 is for fine tuning the frequency.
D2 working as a capacitance diode.

 

[flat5]

jjw, I agree with you. An offset is desirable on receive.

When I get angry with this receiver for it's lack of sensitivity I can call it a homodyne.
That's cool.