Friday, August 23, 2013

the old phase shift question

Actually I am rather flattered by the fact that an experimental design, which I did a couple of years ago, still is under discussion.
The good ole question about how much phase shift you need to run Polyakov mixers for I/Q software defined radio (SDR).
Have a look: http://draaggolf.blogspot.com/2010/01/30m-subharmonic-iq-sdr-receiver.html
Yep, this is a very simple concept design, it worked, it suppressed the lower side-band. And yes, it uses 45 degrees of phase shift at the oscillator frequency, which is half the beat-frequency (for reasons of using frequency doubling Polyakov mixers).
Please see the comments on why other designs, resulting in 180 degrees of beat-frequency shift, still seem to work just fine.
My design is not perfect at all, using a potentiometer to adjust a radio-frequency phase-shift is neither elegant, nor stable. Keep in mind, that this quick and dirty design was more a proof of concept rather than a production ready machine.
73!

Wednesday, August 14, 2013

12m XTAL Controlled QRSS / WSPR / QRP / SDR

In a previous post, I described the use of CD-crystals for 17m weak signal operations. This post is attempting to do the same but for the 12m.
The crystals I am mentioning here are also used in CB-rigs, although harder to find. Here's would be a possible source.

QRSS: 37.340MHz => 12.446MHz x 2 = 24.892MHz
QRP/SDR: 37.350MHz => 12.450MHz x 2 = 24.900MHz
WSPR/QRSS: 37.395MHz =>  12.465MHz x 2 = 24.930MHz

SW+ 40 possible WSPR/QRSS mod

Regular readers of my blog will know that a PSK-Warbler could easily be modified into a 40m WSPR transceiver. I believe that Dave K1SWL even mentioned this mod during his talk at the FDIM-QRP-Convention.
Very unfortunately, the Warbler has been retired later that year. It seems the days for easy 40m transceivers are over.

However, there is hope, the 40m SW+.

The trx employs a 4MHz crystal filter for reception, since the SW+ is a superhet A1A transceiver, there is no if-filter in the tx-train.
The transmitter consists of the VFO, and up-converter (NE612), a link-filter, linear buffer and driver and a class-C power amplifier.

This is what I see, could be done to the rig... please grab a circuit diagram (can be found on K1SWL's homepage).




Concept


We want to have zero-beat at 7038.6kHz. So the sum of intermediate frequency and local oscillator should end up there. With the 4MHz intermediate frequency the kits comes with, we would need a frequency source of either 3038.6kHz or 11038.6kHz. Both are somewhat hard to reach on standard crystals.
Now, lets have a look what get's us close...
Lets check out the options:
  1. 5.000 + 2.048 = 7.048
  2. 5.200 + 1.843 = 7.043
Option one will require penning down the 2.048MHz crystal, however would open the option of using a 2.000MHz crystal for the lower band edge QRSS range. Additionally, a 2MHz ceramic resonator oscillator would enable operation in the regular CW portion of the 40m band.
Option two would probably work by just severely pulling of a 1.843MHz crystal.



VFO


This is were the mod begins. The VFO should be skipped; an external xo will be doing this job. Having the local oscillator external will provide some advantages for QRSS operations. First of all, temperature stabilization should be applied. Secondly, one may consider switching between several crystals.
Most importantly, an extra 7dBm output should be added for a transmit DSB modulator.


BFO

In a side-band transceiver, an offset between the BFO and the up-converter is not wanted. In a CW transceiver, one wishes to have an offset of about 800Hz between RX and TX. Since the SW+ is a CW transceiver, we would like to get rid of the off-set. The easiest way of doing this would be to buffer and feed the BFO into the up-converter.


Receiver

The receiver is pretty much were we would like to have it. All crystals should be changed according to the mod you would like to perform.


Transmitter

This is were the choice of option will become important. Buffer and driver look just fine, however, it is definitely required to convert the PA from class C into class AB for linearity. It might be easiest to just design a linear PA, potentially external, and feed the buffer into it. Another option would be to merge designs. Genesisradio.com.au once sold a qrp-transmitter kit, the Q5, which was equipped with a 500mW class A linear PA. The PA design itself genesisradio took from their SDR-TRX.

Wednesday, August 7, 2013

new blog created

Dear ham radio operator,
Lately I filled this blog, my radio blog, with some computer / IT related posts. This was due to a lack of a dedicated place to publish those posts.
It seems inappropriate to continue with this practice, hence, I decided to create another blog concerned with this sort of technology:
http://homebrew-it.blogspot.com/
Hopefully I will have to report something radio in the near future.
73

ccrrrcrcrc crcrrcccrccc ... SILENCIUM!

My workstation employs a mouse which performance-wise, I really like. It is a super cheap Logitech M100. The only thing that is really annoying is the sound and cheap feel of the scroll wheel.
I know, this is a matter of taste. However, here's one to try for yourself (and this may apply, mutatis mutandis, to other computer mice too!).

The feel and sound is created by a spring mounted against the inner corrugated surface of the wheel (of course, first you need to remove the single screw on the bottom of the device... you know all about this....
overview of the mouse
close-up of the click device
Key point of this very simple mod, compared to other attempts available in the mists of the internet, is to remove the bloody click thing all together.

In order to achieve the goal, the only thing to do is gently slide out the scroll wheel assembly. There is nothing holding down said assembly, just grab it and off it goes. The photograph below shows the assembly, the clicky spring device still in place, whilst the load spring held on the axle just fell off (this spring we actually want to place into its original place when assembling the mouse again!).
the wheel assembly taken removed from the mouse, note the load spring
Now, gently pull the actual wheel from the assembly. The clicky-clacky (ccrrrcrcrc crcrrcccrccc) spring will most likely fall off in the process. Do to the severe complexity of this stage, I forgot to take a picture... sorry for that ;-)

The following step will be to put the wheel back in its original position in the assembly, w/o the crcrcr-spring of course.

As a last step, we slide the wheel assembly back by using its guard rails. Mind the "scroll wheel click load spring"!
now w/o the noise device
Again, I leave it all to you to put the lid on it all and screw it all down by the single mounting means we had to dismantle in the first place.
As a result, I feel personally very positive about the modification. Not only is the bloody noise gone, the now freely spinning scroll wheel provides a real smooth experience.