Hello everybody,
To solve this problem, I am thinking of my old HW101 from Heathkit, there we
had as input filter a tuned preselector, consisting of two tuned coils and a
tube inbetween, that was a good way of making life easy for the first mixer, a
homebrew diode ring. This preselector was used during transmitting as the
bandfilter to get rid of the unwanted sideband. I think if anything else fails
we could think of a modern version of this preselector, unfortunately, it means
a second (analog) tuning knop on the front panel. (Unless we can figure-out a
way to let the processor do this extra tuning).....
As i think of this, i do have an old module from an earlier trx using pindiodes
to switch-in 7 bandpass filters, this might do, if i change the 6 meterband
into 60. I still have the schematic somewhere . (4 and 10 MHz away is far
enough for even the sloppiest filter)
Another way is using one fixed oscillator block and two si570's or do some more
calculations and measurings and come-up with a number of low and hi-pass
filters.
My bet is for now on the si5351 using 2 out of 3, with an SMD oscillator block
of 32 MHz, using 42 MHz as first IF. And the tx bandfilter.
Regards Ben PA9B (in Turkije)
-------- Origineel bericht --------
Van:Ashhar Farhan <farhanbox@xxxxxxxxx>
Verzonden:Thu, 02 Jun 2016 08:55:03 +0300
Aan:minima@xxxxxxxxxxxxx
Onderwerp:[minima] Re: looking for HF1
The ideal case, of course, is that we use the Si5351 for all the three
oscillators. let me take one at a time
1. 1st oscillator : this is essential as the 1st oscillator needs to be
tunable. There is a subtle catch : if there is a leakage of the any other
oscillator into this, the output will certainly show up in the transmitted
signal. we can live with a bad receiver, we will be arrested for a bad
transmitter. so, this is legal stuff.
2. 2nd oscillator : I chose 45 MHz IF frequency as it is easily handled by
conventional HF methods and it is sufficiently above the HF range to be
cut-off with the usual LPF filtering. The oscillator needs to be around 55
MHz. The trouble is that overtone oscillators are very particular about being
tuned. i am looking at ways of avoiding critical tuning. but one main reason
that we should look at using Si5351 for this application is that the overtone
oscillator will be a no-brainer.
3. BFO : the bfo needs to be tunable over a small range to allow for the upper
and lower sideband selection. at present we are using a VCXO to do this.
having Si5351 do this will make things really simple.
now, here is the catch, BFO, if it leaks into the VFO (1st oscillator) will
show up as a strong carrier in the transmit output as it is inside the
front-end LPF's passband. The 55 MHz oscillator, when leaked into the VFO will
beat with the oscillator and produce a response that is consistenly 10 MHz
away from the signal. In fact, I faced this in the PCB that I designed for the
HF1. It works like this : let's assume that we are tuned to 14 MHz, our VFO
would be at 45+14 = 59 MHz. This 59 MHz will beat with the leaked 55 Mhz and
produce a transmission spur at 59-55 = 4 MHz.
So, that is the whole problem set. over to you guys now...
- f
On Thu, Jun 2, 2016 at 10:48 AM, Ben Aupperlee <beninturkye@xxxxxxxxx> wrote:
Hello Joe (and the whole bunch),
We can go for 2 out of 3, with possebly buffering or so, as there are little
SMD blocks containing Xtal and oscillator chip, virtually a one component Xtal
oscillator that can be useful for osc nr 2. Conrad.com sels them for 1 to 3
Euro, single item price. They only do not have the exact frequencies of 55 or
35 MHz, but I hope there are outlets that are better sorted.
But whatever we do here, it will not interfere with the construction and
layout of the motherboard , so that can go on. As long as we agree on the
interface with the motherboard, and stick to that.
I see this oscillator sub-board as a rectangular board, standing on one of the
long sides, not higher than 2 or 3 cm so it fits inside a shielding box, with
some pins in the motherboard, 3 pins for signal and close by 3 ground pins,
then somewhere on one end of the board power supply pins. The clock and data
lines comes directly from the digital board, mounted on the front panel and
housing the processor and LCD. And should be shielded from the analog part of
the set as this whole digital circuit radiates the 16 MHz clock frequency.
That is the reason I like to split the whole set in an analog and digital
part. (With a solid shield inbetween)
Somebody might contact the si manufacturer and ask if they have an application
note for this chip, explaning how to prevent or get around this crosstalk.
I am interested to hear what Ashhar will think of our idear to use the si5351,
as I remember well, he rejected this chip some year ago because of the
crosstalk.
Regards Ben PA9B (in Turkije)
-------- Origineel bericht --------
Van:Joe Rocci
Verzonden:Thu, 02 Jun 2016 03:18:46 +0300
Aan:minima@xxxxxxxxxxxxx
Onderwerp:[minima] Re: looking for HF1
Ben
I don't have a dog in the fight, so I'll defer the modularity and
component-type selection to you. Sounds like an interesting version though. I
did want to point out my personal experience with the Si5351. It's usable, but
not spectacular. Having said that, I have a phasing-method rig on the bench
that uses it and I'm happy with the performance so far. Numbers to follow some
day.
Joe
Sent from my Verizon Wireless 4G LTE Tablet
-------- Original message --------
From: Ben Aupperlee <beninturkye@xxxxxxxxx>
Date: 06/01/2016 7:23 PM (GMT-05:00)
To: minima@xxxxxxxxxxxxx
Subject: [minima] Re: looking for HF1
Hi Joe,
Yes, but we came from a situation where we had an X-Tal oscillator for nr2,
and the idea was: replace it with a chip and save a lot of board space an
discreet components. On the other hand this might solve the crosstalk problem.
Are we going to place a lot discrete components on that small SMD board, or
will we put them on the motherboard and only use the smd board for osc 1 and 3.
Groetjes Ben (in Turkije)
-------- Origineel bericht --------
Van:Joe Rocci
Verzonden:Thu, 02 Jun 2016 02:03:59 +0300
Aan:minima@xxxxxxxxxxxxx
Onderwerp:[minima] Re: looking for HF1
Ben
Oscillator #2 doesn't have to be variable, or even very accurate. A simple
crystal oscillator will do and you can still do passband tuning. I suggest
this because the Si5351 will probably have horrible crosstalk spurious if all
3 oscillators are running. It's much better if you just run #0 and #2 and keep
the output traces well isolated from each other. In the. Si570 approach, this
eliminates an expensive 3rd part.
Joe
Sent from my Verizon Wireless 4G LTE Tablet
-------- Original message --------
From: Ben Aupperlee <beninturkye@xxxxxxxxx>
Date: 06/01/2016 6:45 PM (GMT-05:00)
To: minima@xxxxxxxxxxxxx
Subject: [minima] Re: looking for HF1
Hello Ashhar,
I had some exchange of idears about SMD and modules, so here a new version
with a new drawing, the old .gif is also used in this temp.htm text.
Regards Ben PA9B (in Turkije)
