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7.7 Band Pass receiver input filter characteristics


Pierre Guillot
 

New to this group and interested by this nice QCX+ my last experience soldering components was with the late Heathkits and I need to refresh my knowledge of HamRadio!
Could you give the vertical scale of these graphs? 
May I understand that a QCX+ 20m can receive with a good sensitivity from 9.69MHz to 22.54MHz, possibly by resetting C1?
May I also understand that it can transmit some power on the 30 and 17m bands?
One last question, I plan to buy one kit with the Japanese firmware but could also wait for the QCX-mini, will this Japanese firmware be usable with the mini?
73 to all - FK8IH - Pierre


@CurtisM
 

Pierre

bon jour.  sitting listening to my 80m QCX+ this rig is amazing.  I highly recommend its assembly - wonderful manual and thoughtful design.  the companion 25/50 watt amplifier sits completed - ready for alignment and careful check out. 

the rig and its control firmware are not really intended for multi-band use - but there are examples out there.  the input receive bandpass filter is essentially a high Q single-tuned filter - it needs nothing more.  now the important part - the special quadrature cancellation network combining the unique mixer and dual audio chains only works on one band.  choose your favorite band for your first QCX+

Please find the video tour of QRP-Labs ... and download the manual to see what you think.  You will be thrilled by your build and operating experience - glad to have you join us here.  I am merely a several-times builder of these fine products. 

73 Curt wb8yyy


Hans Summers
 

Hello Pierre
 
New to this group and interested by this nice QCX+ my last experience soldering components was with the late Heathkits and I need to refresh my knowledge of HamRadio!
Could you give the vertical scale of these graphs? 

The graphs in section 7.7 are spectrum analyzer screenshots from my Advantest R3361C spectrum analyzer, via GPIB to USB adapter. The top of the grid represents an insertion loss of 0dB. The vertical scale is decibels with 10dB per division. So the vertical scale goes from 0dB insertion loss at the top, to 80dB insertion loss at the bottom.
 
May I understand that a QCX+ 20m can receive with a good sensitivity from 9.69MHz to 22.54MHz, possibly by resetting C1?

It is optimized for 20m; the range 9.69MHz to 22.54MHz is what I measured as the tuning range using the C1 trimmer. But basically yes, you can tune it to measure in this range. 
 
May I also understand that it can transmit some power on the 30 and 17m bands?

Yes BUT... On 30m the harmonic attenuation will not be so good, and perhaps not good enough to satisfy regulatory requirements. On 17m since 18MHz is likely above the 20m filter cut-off (3dB) frequency, the output power will be attenuated. 
 
One last question, I plan to buy one kit with the Japanese firmware but could also wait for the QCX-mini, will this Japanese firmware be usable with the mini?

QCX, QCX+ and QCX-mini all use the same firmware chip. HOWEVER, there is going to need to be a minor change to the firmware, to make the LED backlight on the QCX-mini controllable by a menu item. If you use current firmware 1.05 then the QCX-mini LED backlight will be OFF. You could hardwire it to be on, of course, via a simple hardware modification. 

All the Japanese firmware version does is prevent operation outside the Japanese band limits. This is a requirement for the Japanese license, strictly speaking, though many Japanese amateurs are using the normal QCX firmware. There is nothing else about the firmware version that is Japanese. I.e. the menus are still in English and the Morse is still international Morse etc. Only the frequency restrictions are implemented. The penalty for this is that the WSPR functionality is disabled - this is necessary in order to make space for the frequency check. 

At the moment the allowed operating frequencies, with the Japanese firmware version, are:

3500000 - 3575000
3599000 - 3612000
3680000 - 3687000
3702000 - 3716000
3745000 - 3770000
3791000 - 3805000
7000000 - 7200000
10100000 - 10150000
14000000 - 14350000
18068000 - 18168000
21000000 - 21450000
24890000 - 24990000
28000000 - 29700000

I have in mind some dim recollection of someone telling me the 80m allocations have been changed recently; if anyone can confirm this and what the new band limits are, I can update that in the firmware. 

73 Hans G0UPL
http://qrp-labs.com


Ben Bangerter, K0IKR
 

Curt, in his post, stated:
 “... now the important part - the special quadrature cancellation network combining the unique mixer and dual audio chains only works on one band.”

The Tayloe detector, audio combiners and amplifiers, audio phase shift networks, audio bandpass filter and audio output amplifier are the same in all frequency versions of the QCX/QCX+, essentially everything following the rf transformer In the receiver circuit.

73, Ben K0IKR


Pierre Guillot
 

Hello Hans,
My last question was due to my plan to use a QCX+ or mini in Thailand. That country prohibits transceivers which can by construction transmit outside of Ham bands limits. The local limits are the same as in Japan except the 80m band limited from 3.5 to 3.6Mhz.


@CurtisM
 

Ben

i think my point is that these adjustments are made to relate to the specific transformer windings for the intended band - so they should not be expected to null well on more than one band, with a single set of adjustments.  so while there are some folk experimenting with their QCX across multiple bands - even into unusual antennas yet (-:  - but the design intent is a fine single band rig.  not to mention that the PA is similarly tuned in its design for a single band.  folk are welcome to experiment. 

73 Curt