From: Gordon Gibby KX4Z via groups.io

Sent: Monday, December 30, 2024 9:30 AM

To: BITX20@groups.io

Subject: Re: [BITX20] Zbitx

 

I wish I knew as much about radios as Ashhar does!

 

When I was a Novice, way back in the 1970's, I was concerned over the difference between this radio with 0.5 microvolt "sensitivity" and that one with 0.35 microvolt sensitivity -- completely clueless that the AMBIENT NOISE on the HF bands from all sources combined, was far far far greater than these values.   

 

The noise factor gives a decibel (logarithmic) ratio that demonstrates how much the (receiver) amplifier degrades the pure signal to noise ratio of the laboratory input.   For VHF/UHF work (where there is much less background noise) one would prefer more like 3-4 dB NF.   For HF, AB4OJ (https://www.ab4oj.com/icom/nf.html ) calculates that if you can get below 16 dB you'll be fine, and he states:

 

"To ensure that it is always externally noise-limited, an HF receiver system, located at a quiet radio site and operating at the higher end of the HF range, should have F < 16 dB. This is equivalent to a -158 dBm/Hz noise floor or a sensitivity spec. of -113 dBm (0.5 µV) for 10 dB SINAD (equivalent to a -123 dBm noise floor) in a 3 kHz bandwidth."

 

In the presented schematic, Ashhar is using a very simple ring-diode input mixer as his first stage, which gives him the dynamic range he wanted.   If you drop down to about page 14 or 15 in this text on MIXERS, you can read the pro's and con's of double balanced ring diode mixers:   https://www.qsl.net/va3iul/RF%20Mixers/RF_Mixers.pdf    

 

Ashhar is using BAT54S surface mount shottky diodes with two diodes per device, likely to get the best matching possible without hand selecting (possibly far better than hand selection!).    The Vishay device also includes the cryptic note:  

These devices are protected by a PN junction
guardring against excessive voltage, such as
electrostatic discharges

(which sounds helpful to me!   I prefer that if I'm carrying this radio out into the boonies, the more protection against Bad Things, the better!)

 

Stealing some of Ashhar's work, a few years back I put together a small PCB for some 6th graders to solder up their own direct conversion DSB receivers.  We literally used individual glass-envelope diodes and I was amazed at what those receivers could pick up!!   Right from the antenna into the ring detector with an injected switching system from an Si5351.      The youngsters had GREAT FUN SOLDERING....and were less fascinated by the world of received signals.....   

 

In addition to being adequately sensitive for the RF environment you're working in, you prefer that your receiver isn't OVERLOADED.   Every design is a compromise....if you want absolute best in ALL CATEGORIES, be prepared to need a forkloader to go camping and $10,000 to purchase!    Fascinating data about the dynamic range of various supposedly-great receivers has been gathered and catalogued, and makes for fun perusing to see how crappy were the receivers I used as a lad!   http://sherweng.com/table.html

 

The 5-watt world isn't (currently) my kettle of fish, but we have folks in our group who really dig this QRP stuff.   To see something out there under $200 is probably causing a LOT of consternation among a few manufacturers.   A year or so ago when Ashhar come out with the sBitx I told our little group of hams in Alachua County Florida that this new sdr technology was a game changer, and it looks like that is becoming ever more true!  

 

Gordon KX4Z

 

 

OK 

1.  I’m assuming you were able to adjust the idle current with the bias potentiometer and get expected results. If not, then there could be a DC disconnection somewhere in the power amplifier stage.

2.  Those four metal cans to the left of the power amplifier stage are the driver stage. They should get pretty warm or even hot within 15 seconds if you put the radio into any single side band mode and key the microphone. Even if you don’t say anything. If they don’t, then there is some problem with the DC connections to the driver stage.

3.  Do you have any other means to measure the output of the transmitter? A SWR meter or anything similar? If so, try to get some kind of a read on whether there’s any real power coming out even if it is only 100 Milli Watts or so.   

Do you hear anything in another receiver tune to the same frequency? Usually, you’ll hear something if there’s any real power out put

4.   Do you have a voltmeter?

On Dec 30, 2024, at 15:00, bretnemeth via groups.io <bretnemeth=yahoo.com@groups.io> wrote:



It receives fine. I can hear everything that it should with an antenna.

 

When I transmit in any mode I see that it looks like it is transmitting on the screen but it shows 0 watts and 1.0 SWR. Doesn't matter what mode I transmit on. 

That's a bit over my head. I would rather buy a new board than go that deep. I'll dig around a bit but if it requires equipment I don't have it's over.

One of the legs broke on one diode. So I ordered more. Only problem is I put everything back together and set the bias. Still no power out. Starting to think I blew another fet or maybe there is a software problem. I have read about some peoples radios not working due to software.