Re: Freq-counter


Thank you ,both Arv and Michael, for your assistance with my frq-counter.Now it works 100% and now i will go further to the TX-part.
best 73 de sm6mpa/Hasse

--- In BITX20@..., Arv Evans <arvid.evans@...> wrote:

Hasse - SM6MPA

If I am understanding this correctly, it seems that the problem may be in
getting the counter clock exactly on-frequency, instead of a problem with
the BFO versus IF filter passband.

This can be verified by simultaneously using both the BITX counter and
your ICOM 726 to measure frequency of the BFO. If they agree, then the
BITX counter clock would be matched with that of your ICOM. If not,
then one of the two counters is off-frequency and needs to be adjusted.

If the problem then comes back to being the BFO versus the IF passband
you can perform this type of BFO alignment procedure:

1. Turn the transmit level control down to zero output. This protects
the RF PA section while you are keyed up for long periods doing

2. Unbalance the BFO mixer, key the transmitter, and measure RF
voltage (use a detector probe or oscilloscope) at the collector of
Q-10 (2nd Transmit IF Amplifier).

3. Very slowly tune the BFO across the filter passband, making and
recording measurements at 200 Hz intervals.

4. Plot these voltage measurements versus frequency on graph paper.
This will give you a fair idea of the filter shape. /

NOTE: this is a voltage graph, and not a DB chart, so it will look
rather rough. The higher voltage end is emphasized and will
over-state any ripple in the filter passband./

5. From the graph you should see two points where the voltage drops
to 1/4 what it was at top of the passband. These are the -12 db

6. The BFO can be adjusted to set at the lower frequency -12 db point.
This provides adequate passage of the USB signal, and helps by
providing an additional 12 db of carrier suppression beyond that
which is provided by the balanced modulator.

7. Re-balance the BFO mixer, and re-adjust the RF PA section drive

The crystal filter will probably not be exactly on 11.0 MHz. Usually
the center of it's passband is slightly lower in frequency. This is the
reason you need to adjust the BFO to position it at the lower -12 db
point. If the normal pitch of your voice is quite low, you may want
to position the VFO somewhere between -6 and -12 db on the lower
skirt of the filter passband. But, for those with normal voice pitch
the -12 db point seems to work.
For the purists:/*
While random selection of 11 MHz crystals will provide an adequate
filter shape, those who want to play and to optimize their filter can
swap the crystals around until they have the best possible filter
shape (flat on top and a steep side on the lower frequency edge).
This involves making many measurements and several graphs.
It helps to number your crystals so you don't get confused with
which crystal was in what position. You can also play with the
capacitors involved in the crystal filter. C-62 and C-53 affect
transmit side impedance, and C-52 and C-89 affect receive side
impedance (there is some interaction between receive and transmit
side capacitors). Impedance mismatch causes ripple across the top
of the passband. Capacitors to ground between the crystals affect
bandwidth (increase C for less bandwidth). C-55 should always
be larger than C-54 and C-56, but not always double their values.

Those who are adding CW capability to their BITX may be interested
in that last paragraph because it shows a way to obtain narrow IF
passband by switching in additional capacitance in the crystal filter
when in CW mode. This does change the filter impedance, but for
narrow-band CW work any additional ripple across the filter top is
not as much an issue as it might be for wider bandwidth modes.

Okay...I seem to have answered more questions than you ask, but
hopefully this will provide some additional information that could
be helpful for optimizing your BITX20A, and for others who are
working on similar efforts.

Arv - K7HKL

On 05/24/2010 10:51 AM, Hasse wrote:

Hello Arv !
This is a cosmetic error,but i will have the correct frequence on the
display.I cannot get 11.000Mhz with the c88(33pF) on pcb.If i resolder
it there is no problem at all to get 11 MHz,when i trim c72.BUT ! the
audio sounds terrible.I change the c88 to 22pF and the max BFO freq.
i obtained was 10.99618 MHz.The rx and audio is very good here,but than
the problem remains with wrong reading on the display.It shows 8.82Kz
to much.I have an idea to change the chrystal to 11.00382 MHz (?)
Best 73 de sm6mpa/Hasse

--- In BITX20@... <>, Arv
Evans <arvid.evans@> wrote:

Hasse SM6MPA

Use your Icom 726 or Atlas 210 to check the frequency of your BITX20A
BFO and it's VFO.
Then do the math to see what frequency it should be receiving.

The problem you described is usually associated with having the BFO not
properly aligned
with the crystal IF Filter. If you change the BFO frequency you need to
re-measure the BFO
with the digital display so that it will compensate for that change.

Arv - K7HKL

On 05/22/2010 03:31 PM, Hasse wrote:

Hi there !
Now i`m ready with the RX part of the BITX20A kit, and i have
and connected the freq-counter to the kit.(to Q6 via 22pF).I have very
good rescieving signals, thats not the problem,the problem is that
counter shows 5KHz to much.On my icom726, and Atlas 210 the station
was on 14.285 , but my BITX shows 14.290.In what mode can i use the
trim-cap (the green one) on the freqcounter-pcb ?It seems to have no
effect at all when i adjust it.Any ideas on the forum ?
best 73 de sm6mpa/Hasse

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