Try to learn something about everything, and everything about somethingThomas Huxley “Darwin's bulldog” (1824-1895)

Trace: • Laptop FreeBSD • HF / VHF Xgate • Kenwood TM-D710 • Comparison - The Thief Of Joy • The Nodes • R Pi Image files • Casio F91W • QRSS • 60m Transceiver • OFCOM

You are here: Make It Tea » Public Area » Radio » Radio Topics » Homebrew Projects » 30m Phasing Rx

Differences

This shows you the differences between two versions of the page.

Link to this comparison view

Both sides previous revisionPrevious revision
Next revision		Previous revision
public:radio:topics:arduino:30m_phasing_rx [22/07/26 11:35 BST] – [Updates] johnpublic:radio:topics:arduino:30m_phasing_rx [28/07/26 14:23 BST] (current) – [Mute clicks & thumps] john
Line 6: Line 6:
 I recently found an old project that had been left in the junk box unfinished. It's a direct-conversion phasing receiver for 30m. I recently found an old project that had been left in the junk box unfinished. It's a direct-conversion phasing receiver for 30m.
  
-Inspired by my current binge-listen to the Soldesmoke Podcast [[https://www.soldersmoke.com/]] archives (I'm starting at the beginning and listening to them all.. currently I'm at around Episode 30) I thought I'd restart the project and build it into the planned-for 30m CW transceiver. There's a 5W PA strip that's recently been used in my [[qrss_tx| QRSS MEPT]] but I'll build a new PA for the MEPT and use the original one for it's original purpose.+Inspired by my current binge-listen to the Soldersmoke Podcast [[https://www.soldersmoke.com/]] archives (I'm starting at the beginning and listening to them all.. currently I'm at around Episode 30) I thought I'd restart the project and build it into the planned-for 30m CW transceiver. There's a 5W PA strip that's recently been used in my [[qrss_tx| QRSS MEPT]] but I'll build a new PA for the MEPT and use the original one for it's original purpose.[[..:..:..:blog:new_tx_finished|]]
  
 I repaired a few broken wires and dry joints and the RX is working. I've checked that the phasing opposite-sideband rejection is working and given it a tweak to check that the adjustments are doing the right things. All looks good. I repaired a few broken wires and dry joints and the RX is working. I've checked that the phasing opposite-sideband rejection is working and given it a tweak to check that the adjustments are doing the right things. All looks good.
Line 57: Line 57:
 ===== Updates ===== ===== Updates =====
  
-After using it alongside my separate 30m TX [[30m_cw_tx|]] while testing the TX I found it **very** necessary to turn the volume down **FULLY** before transmitting, otherwise, un-muted, the receive audio is dangerously **LOUD**... that's fine as long as I remember to turn the volume down.... but after forgetting once, while wearing headphones, I don't dare risk it again!+After using it alongside my separate 30m TX  while testing the TX I found it **very** necessary to turn the volume down **FULLY** before transmitting, otherwise, un-muted, the receive audio is dangerously **LOUD**... that's fine as long as I remember to turn the volume down.... but after forgetting once, while wearing headphones, I don't dare risk it again!
  
 I added a simple audio switch in series with the audio path, between between the post-BPF amplifier stage and the "top" of the volume pot. I added a simple audio switch in series with the audio path, between between the post-BPF amplifier stage and the "top" of the volume pot.
Line 69: Line 69:
 It works well enough to both save my hearing and to provide a useful sidetone from the receiver during transmit, which means I don't need to add a sidetone circuit. It works well enough to both save my hearing and to provide a useful sidetone from the receiver during transmit, which means I don't need to add a sidetone circuit.
  
-{{:public:radio:img_20260722_113212.jpg?400|}}+{{:public:radio:img_20260722_113212.jpg?direct&400|}} 
 + 
 +==== Mute clicks & thumps ==== 
 + 
 +I found this arrangement of using diodes to "share" the pull-down transistor switch with the c/o relay in the 30m TX [[..:..:..:blog:new_tx_finished | Blog post about TX ]] to perform poorly after I'd used it for a while and realized it needed a bit of a tweak to make it "right" rather than "okay". 
 + 
 +I noticed a loud click/thump as the first CW character was sent and the mute operated. Initially I assumed that the MUTE wasn't operating soon enough, or that the QSK wasn't delaying the SI5351 VFO enable line sufficiently long enough to allow the MUTE (and C/O relay) switching. Much probing with the 2-channel scope shows that the relative timing is okay. The loud initial click occurs whenever the first loud audio feeds into the mute circuit, regardless of how long the mute has been active, and how long the RF is delayed. I played with the Arduino code to delay the RF enable so that the RF didn't come on until 100mS after the MUTE and Relay c/o - enough to miss several morse characters - and yet still there was a loud click when the first loud audio arrived. 
 + 
 +I tried pulling the MUTE line down to ground with a croc-clip-lead - and this cured the problem.  
 + 
 +Looking at the MUTE line on a scope shows that it normally only goes down to about 1V when the transistor switches, not 0V and this leads to an initial loud click when the first loud audio arrives on first key-down. Once the click is gone the mute behaves correctly, but it appears that unless the MUTE line is hard-down it doesn't fully mute until a few mS after the first audio comes on the input to the SD7000. 
 + 
 +Sharing the MUTE line via a single transistor switch and steering diodes was the problem. 
 + 
 +The cure was to use a separate 2N3904 transistor to pull the MUTE line down hard without a diode inline.  
 + 
 +Now the MUTE operates correctly, and smoothly and the faint leak-through is enough to act as a simple sidetone. And no loud CLICK on key down. 
 + 
 +==== RX Bandpass Filter ==== 
 + 
 +Initially I installed a simple 2-stage Op Amp BPF. It was designed several years ago and I don't know what my though process was at the time - what Q and Gain I was aiming for. It's not //bad// but it's not as good as I would like it to be. 
 + 
 +I had a look at the component values and reverse engineered the design. 
 + 
 +With R1 = 47k, R2 = 6k8, R3 = 68k and the capacitors 10n it seems that the design Q was only 1.7 and the gain 0.7 and center frequency 800Hz  . With two identical stages this would mean a designed overall gain of about 0.5. 
 + 
 +I'm sure it could/should be better. 
 + 
 +I redesigned for Q = 4 & Gain  = 1.1 and Frequency = 700Hz, keeping the 10n capacitors 
 + 
 +  * R1 = 82k 
 +  * R2 = 2k7 
 +  * R3 = 180k 
 + 
 +I breadboarded this to test and it gives a Q and gain just less that 1 with much better shape and attenuation of HF signals. 
 + 
 +I then re-designed again for Q = 5 and got FIXME from memory 
 + 
 +  * R1 = 100k 
 +  * R2 = 2k 
 +  * R3 = 220k 
 + 
 +This was slightly tighter and works nicely on the bench fed from a signal generator, measuring output on a 'scope. Next I need to build it on veroboard and install it in the RX and compare the performance. 
 + 
 ===== Further Information ===== ===== Further Information =====