Surface Mount Part Bench Work -- it's a skill we'll likely need to hone

Greets!  Oi!

We get many views from Brazil -- a cheerful greeting to these readers.

We're busy at work on the main RF board for our hopeful 5 VDC Direct Conversion receiver.

The RF board is nothing special  -- but it does have a frequency doubler right on the piece
of copper clad board. 

 

Above — The 40 Meter band RF board. We''re evaluating a 5 volt op-amp for this receiver.
The LMV796 in single | LMV797 in dual. Only available in SMT/SMD and low-cost for pretty decent performance. This means we've got to up our game in surface mount technique.  We're assembling a small, reflow station -and- hope to show it off next month if all goes well.

Hybrid through-hole/surface mount works OK -- but soldering really tiny surface mount proves difficult and messy for us. Hence we hope to add a tiny reflow set-up to our arsenal.

Above — A LMV796 next to its prototype board. We hand soldered it -- but its getting tough for us old folks.

 

Above — The hand soldered board with flux spatter galore. The LMV796 datasheet says that with a 10K output load, we can take it to within 25 mL of the rail. Our + rail = 4.99 VDC, so we'll set it up as a simple, non-inverting amp on a scrap of copper clad board to briefly test this specification.

Above —Driven with a 1 KHz home-built signal source -and- pushing it nearly to the rails. 10K resistor load AC coupled to the output. 

Above — We rolled back the signal drive just until the sine wave cleaned up by eyeballing the sine wave in time domain. It's about 22 mV from the rail. 

 
Above — Changeover to FFT. All harmonic tones are under 60 dB down when driven pretty close to the rails -- wow, this seems good to us. The noise performance is similar to the NE5532 which we normally use for higher DC rail voltages. The NE5532 doesn't function well @ 5 VDC and less however.  We seek an inexpensive low-noise, low voltage SMT/SMD op-amp and this part might work for us --- can't wait to try it in a real, future receiver.

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In design work -- we're trying to sort out the buffer for the frequency doubler. We're currently testing out buffer amp options for the frequency doubler. 

Above — We adapted the Q1-Q2 stage of KO4BB, a full-wave rectifier that works like a charm.  Bias gets done with a standard voltage divider --- and VCC = 5 volts. The buffer is a bog-standard push-pull emitter follower.  Wideband matching it to 50 Ω was our goal -- however, first we felt a strong compulsion to first try a narrow match it with an L-network. We found the network values on the bench by just watching the response to different L values and peaking with a 6 - 450 pF variable capacitor. 

The output was connected to a return loss bridge , while the input was terminated with a 50 Ω resistor. Our tune-up frequency was 7.1 MHz. We found the perfect inductance, removed and measured this coil-- and then soldered it back in. We also removed & measured the perfectly peaked variable cap ( 341 pF ) -- a 330p plus 5-60 trimmer cap were substituted. The network was again re-peaked and the results lie in the table. We show the return loss from 7.00 to 7.40 MHz. Clearly, this is a 7.1 MHz L-match!

Terminating  the emitter follower for a wideband match to drive the LO part of a diode ring mixer proved vexxing. Some old tricks like a series resistor failed to give us good return loss results.

We'll keep working on it --- a new buffer was designed on paper tonight --and hopefully it might help move us us in the right direction. Stay tuned.

 Best!