EMRFD Message Archive 9568

Message Date From Subject
9568 2013-12-20 09:25:43 bob_ledoux Questions Re: Strange Local Oscillator Wave Forms

I've been reviewing the local oscillators found in common home built superhet receivers. These are crystal controlled, using a crystal of the same frequency as found in the crystal filter.


The local oscillator is typically a Colpitts design like that in EMRFD figure 4.23. The oscillator consists of a single NPN transistor with output taken from collector or emitter. There is no buffer amp stage.


These oscillators tend to pull some power because they often drive a diode ring product detector requiring about 7dbm.


Building several of these I've found the following:


First; the series Colpitts caps need to be properly sized else the oscillator runs at third or even fifth harmonic.


Second; the wave form is very “un-sine wave like.” I typically see more of a triangular waveform or even a stair-step wave form.


This wave form is perplexing until I consider the product detector environment: The crystal filter will allow a frequency spread of a few hundreds of Hertz to pass. The product detector must separate out the widely spaced IF and AF frequencies. Even a square wave oscillator should perform this task.


Have I got this right? Is the local oscillator wave form immaterial?

9569 2013-12-20 12:10:09 Graham / KE9H Re: Questions Re: Strange Local Oscillator Wave Forms
Bob:

You want the cleanest signal possible as the LO input for the ring mixer,
unless you have appropriate preselection ahead of it to reject all
the L.O. harmonic mixing products that will occur.

Once these signals created by L.O. harmonic mixing with ??? at the
higher frequencies are created in the ring mixer, they fold on top of
your desired signal, and there is no way to filter them out.

With real good higher frequency rejection, you may not care and can even
drive the ring mixer L.O. with a square wave.

As far as the distortion, sometimes it is good to figure out where it
is coming from, because it is probably a "choke point" in your chain
and if you fix or improve it, you can get a higher level out of the chain.

--- Graham / KE9H

==


9570 2013-12-20 14:39:58 William Carver Re: Questions Re: Strange Local Oscillator Wave Forms
Even harmonic content in the diode mixer LO will produce mixer products
on the same frequency as third order IMD. Asymmetrical LO shapes are not
good! PERFECT square wave has no EVEN harmonics, so it's OK, otherwise
Graham's comment that you need to filter the harmonic content out is the
way to go.

W7AAZ
9571 2013-12-20 15:53:32 bob_ledoux Re: Questions Re: Strange Local Oscillator Wave Forms
I understand what you are saying when listening to a modulated signal. But is this true for a CW receiver?  I'm thinking about the carrier passing through a narrow bandwidth crystal filter.  The beat oscillator provides a tone based on presence or absence of the carrier. 

How would the speaker sound different for a CW signal if the beat oscillator is not clean?  I'm thinking about the non-linear effects of mixing two signals (BFO and IF).  If the mixer were linear, the speaker would take on the sound wave of the BFO.  So a square wave would have a harsh sound and a sine wave would have a pure sound.

It sounds like time on the test bench is needed.  One test is worth a thousand expert opinions.
9572 2013-12-20 16:34:53 William Carver Re: Questions Re: Strange Local Oscillator Wave Forms
Your intuition is right: it doesn't matter if there is a single CW
signal in the passband. It'll sound just fine.

However when there are multiple signals in the passband, and harmonics
in the LO, there can be that fall in the audio passband.

When SNR is low, even when there isn't multiple discrete signals in the
passband there are static crashes and noises that make things sound
"mushier" when that m*signal1 +- n+*signal2 process is allowed to take
place.

W7AAZ


On Fri, 2013-12-20 at 15:53 -0800, bobledoux@proaxis.com wrote:
>
> I understand what you are saying when listening to a modulated signal.
> But is this true for a CW receiver? I'm thinking about the carrier
> passing through a narrow bandwidth crystal filter. The beat
> oscillator provides a tone based on presence or absence of the
> carrier.
>
> How would the speaker sound different for a CW signal if the beat
> oscillator is not clean? I'm thinking about the non-linear effects of
> mixing two signals (BFO and IF). If the mixer were linear, the
> speaker would take on the sound wave of the BFO. So a square wave
> would have a harsh sound and a sine wave would have a pure sound.
>
> It sounds like time on the test bench is needed. One test is worth a
> thousand expert opinions.
>
>
>
9573 2013-12-20 17:08:08 William Carver Re: Questions Re: Strange Local Oscillator Wave Forms
TYPO: meant to say

> However when there are multiple signals in the passband, and harmonics
> in the LO, there can be spurious tones/noise that fall in the audio
> passband."

W7AAZ
9574 2013-12-20 21:24:10 Russell Shaw Re: Questions Re: Strange Local Oscillator Wave Forms
9575 2013-12-20 22:38:24 William Carver Re: Questions Re: Strange Local Oscillator Wave Forms
"as long as the zero crossings are clean and correct"
.............is the same as saying "no odd harmonics"

W7AAZ




On Sat, 2013-12-21 at 16:24 +1100, Russell Shaw wrote:
>
>
9576 2013-12-20 22:58:56 Russell Shaw Re: Questions Re: Strange Local Oscillator Wave Forms
9577 2013-12-20 23:38:37 Russell Shaw Re: Questions Re: Strange Local Oscillator Wave Forms
9578 2013-12-21 09:35:29 William Carver Re: Questions Re: Strange Local Oscillator Wave Forms
Yea, I meant no even harmonics..........

A sawtooth, with an adjustable threshold on a squarer or the mixer
switches, gives you adjustability for perfect balance if switch on/off
times differ. Hard to get adjustability if you start with a square wave
(Si570 for example).

It's tricky to get everything perfect, but if your goal is lowest inband
IM it's something that has to be done. In the case of a commercial diode
mixer with well matched diodes you can get a result very close to
perfection by lowpass filtering...with an elliptic or other filter that
gets the second harmonic way down...and overdriving the mixer. The
overdrive diodes do the "squaring" function for you.

W7AAZ


On Sat, 2013-12-21 at 18:38 +1100, Russell Shaw wrote:
>
>
9583 2013-12-23 06:32:17 nm0s_qrp Re: Questions Re: Strange Local Oscillator Wave Forms
A distorted oscillator output is generally the result of too much positive feedback in the circuit.  In a Colpitts circuit, this can be reduced by incrementally increasing the base-emitter capacitor the circuit to the point where oscillation still commences reliably.

73  Dave NM0S