[H]: ABX / BFO Problem: Abgleich des SIERRA

Hi Gang,

I am finishing my second band module for my SIERRA (WILDERNESS RADIO Version Rev. D). Bought 1997 - Build 2015/6.
Right now I am aligning the RX stage. I am using this task to work on my SIERRA (here the ABX), too.

Referring to a post and a document of Richard K3KRW (attached to this post), I needed to adjust the ABX function in my SIERRA, too.
As I don’t have the same software Richard used, I thought I might use what I have. An old version of Spectrogram. To my opinion Spectrogram V. 5.1.7 is doing quite the same as the digital modes software DM-780 Richard used.

Spectrogram V. 5.1.7 is also used by Hams aligning their K2s for their SSB and CW reception.
So why not using it for the ABX topic on the SIERRA.

I now have some questions on adjusting the BFO / ABX on the SIERRA using a spectrographic software connected to the audio output.

-> TEST SETUP:

Connected a signal generator (ELECRAFT XG3) on the desired frequency to the SIERRA input.
Tuned the SIERRA to the same frequency.

Using the PC software Spectrogram V. 5.1.7 bandwidth settings from 0 - 3000 Hz.
Audio 3/4 open.
On the PC I can now see the maximum of the rx signal at about 600 Hz (varies with RX frequency). When I turn the VFO up or down this rx signal moves up or down on the bandwidth scale.

In parallel I have an audio speaker connected.

The AGC is checked and set to about 1/2 position.

C1 and C2 on the band module are set to maximum reception.
This is visible in Spectrogram V. 5.1.7
The BFO is set to 600hz with its maximum, too.

-> PROBLEM:

The ABX is set to wide (0% = max))
Everything is perfect, max. RX signal on Spectrogram V. 5.1.7 and in the speaker.

ABX set to 1/2 (50% of max):
The RX signal strength starts to drop.

ABX set to narrow (100% of max)
ABX to minimum bandwidth, the incoming RX signal (using the XG3 set to -1uV = S1) totally disappears.

I thought when the BFO is adjusted right there will be no loss of the rx signal.

I still think I did something wrong but I have no idea where I took the wrong turn….

How can I manage that the rx signal keeps up, like Richards, even with the ABX fully turned to 100% (narrow bandwidth)?

Any thoughts….

Thanks


72/3 de VY1QRP / DM5TU - sTef

Für die unter Euch die es interessiert hier in paar Antworten von Wayne, N6KR.
Gelöst ist die Situation noch nicht.


Hi Stef,

Maybe you're using the wrong value resistor somewhere. If you had a resistor that was too small, then adjusting the pot wiper all the way to ground would greatly attenuate the signal.
Or you could have a diode backwards, so that when the pot is applying maximum voltage, the diode looks like a small R to ground.

Wayne

-------------

Hi Wayne,

It is the original 10k pot installed as bought in 1998 to the original WILDERNESS RADIO kit.

http://www.elecraft.com/TechNotes/ABX.html


The pot is placed on the front panel and connected on the main board as described in the manual. R31 is replaced by this 10k pot. The pot is labeled B10K.

The only diode close to the ABX and xtal filter is D4 a varactor Diode MVAM 108.
This one is correctly placed.

In the IF amp L2 is correct with 8 turns. R26 between the R31 (10K) and U5 is 20 Ohms.

sTef


Hi Stef,

The bypass capacitors in that circuit are C7 an C8 (both .01 uF).

All of the components should be mounted as close as possible to the crystals. The only thing that can be far away is the pot.

If the filter bandwidth is adjustable with the pot, but dies at one end, it could be that the crystal Q is on the low side. In the filter that would result in lower loss. In the crystal oscillator (X5), oscillation could stop, in which case you'd get no output from the product detector.

Wayne

-----------------
QUESTIONS:

-> Do we now talk about the quality of the filters design, or what are we talking about now?

It's a simple design that worked in all cases I tried.

-> Why am I losing -12dbm when the filter is 100% closed?

Because the crystals and/or varactor diodes are not well-matched. The crystals have to have exactly the same series-resonant frequency, and the varactors' capacitance must track reasonably well with voltage.


Wayne

Ahoi Michael, Hallo Zusammen,

habe nun folgenden Sachstand zu melden:

Habe die Cs Rs im Filterzweig des ABX Kreises überprüft.
Ein paar lötstellen nachgesezt.
Der 10K Pot mal gegen einen 5K Pot getauscht.
etc...

SETUP:

Software wie im ersten Post beschrieben.

ELECRAFT XG3
F = 7.030 Mhz
107dbm
1 uV = Signalstärke ca. S1


Softwareeinstellungen:
Siehe Bild 5 im Anhang


FAZIT:

Ich habe nun einen, für mich brauchbaren, Einstellungskompromiss gefunden.
Das Eingangssignal von 1uV wird bei geschlossenem ABX (schmalstes Filter) um -12dbm abgeschwächt. (-15dbm auf -27dbm).

Seht Euch hierzu mal die vier Bilder in deren Reihenfolge, wie ein Daumenkino an.

Der Marker auf den Bildern steht bei 600 Hz

1. Stellung:
ABX offen -15 db 7030 107dbm 1uV

2. Stellung:
ABX halb offen -15 db 7030 107dbm 1uV

3. Stellung:
ABX 2/3 offen -24 db 7030 107dbm 1uV

4. Stellung:
ABX geschlossen (schmalstes Filter) -27 db 7030 107dbm 1uV

Ich denke das ich damit gut arbeiten kann.
Mich würde allerdings eine Vergleichsmessung eines anderen OM interessieren.

Der im 1. Post genannte OM Richard, K3KRW beschreibt, dass er keine Verluste in db bei geschlossenem Filter feststellen kann.
Er definiert allerdings auch nicht das Eingangssignal.

Schicke ich -73dbm bzw. ein S9 Signal in den RX des SIERRAs, bleiben die Verluste bei geschlossenem ABX vernachlässigbar gering.

FRAGE:
Sprechen wir hier nun von der Güte des Aufbaus der Filter, oder von was reden wir hier nun?
Warum verliere ich -12dbm wenn das Filter ganz zu ist?

Crystal Q - die Güte?

Beste 72 und DANKE für zurückkommen

sTef

DM5TU / VY1QRP