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R:190720/0944Z @:GB7CIP.#32.GBR.EURO #:12948 [Caterham Surrey GBR]

From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW

By G8MNY (Updated Dec 04)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

Being involved with ham training, I have been doing all the techical demos including AM & FM (Sigen), but I was ask to do a SIMPLE demo of SSB generation which my siggen can't do. As I only have a wide 50kHz bandwidth filters in my spectrum analyser scope adapter & wanted to show the sidebands etc, normal comms SSB is out, so I came with this set up....

┌───>SCOPE AF──────────────────────────────────────────────┐
│ ┌─>SCOPE DSB & AM─────────┐Y1 │Y2
│ D1 │ ┌────────┐ ┌───────┐
│ ┌──┤<├──┬─────>o─┴─68Ω─o<───┤SPECTRUM├─>X │ SCOPE │ ┌──────┐ u1 │ │ |) o┐ ┌o │ANALYSER├─>Y │DISPLAY│ │AF OSC├─┤├─┬──┴──4k7──┬──┤ T1 |)──┐ _│_ _│_ │ ADAPTOR│ └───────┘ │0 - 2V│ 4k7 === │ |) │ ┌---------┐ │ 1-90MHz│
│100kHz│ │ 1n│ └──┤>├──┘ │ └---------┘ └────────┘
└──────┘ 0-5V ┌─────┴────┐ D2 │ │ │
DC │RF SIG GEN│ ├────┴─────┴──
│ 21.4MHz │ _│_ 21.4 MHz
│ ±100kHz │ /// XTAL FILTER
└──────────┘

I cheated with 100kHz for the Audio to solve the analyser display problem. I initially built from junk on just 2 BNCs, but I soon boxed it up. Diodes D1 & 2 are any small silicon type, I did find proper mixer types better (more linear) but could only handle low level, but my scope was happier with larger signals.

T1 is a medium sized ferrite bead 6mm dia with 2x 8 turns (bifil) wound on it with the centre tap (opposite end of each wire) earthed.

The filter I had is a 7.5kHz wide one (narrower than the spectrum analyser resolution) & it is totally unmatched, but works well with only a 68Ω to equalise the loss for filter bypass (DSB) mode.


SPECTRUM DISPLAYS

CARRIER │ AM │ DSB │ │ SSB │
Y │ │ │ │ │ │ (upper or)│
│ │ │ │ │ │ (lower) │
──────┴────── ─┴─┴─┴─┴─┴─┴─┴─ ─┴─┴─┴─┴─┴─┴─┴─ ────────┴───── X
| | Mixer|/ \ Carrier
Over Mod Distortion Nul
Products

SCOPE DISPLAYS
/'\ /'\ /'\ /'\ /'\ /'\ AF------------- AF | | | AF | | | AF | | |
Y1 \./ \./ \./ \./ \./ \./

_ _ _ _ _ _
RF░░░░░░░░░░░░░ RF/' `\._./' `\._ RF/ \./ \./ \./ \ RF░░░░░░░░░░░░░░░ Y2░░░░░░░░░░░░░ \._./' '\._./' \_/'\_/'\_/'\_/ ░░░░░░░░░░░░░░░
|
Crossover
Nuls

AM can be displayed by off setting the balance of the mixer with DC.

I found the Scope display worked best with 1-10kHz AF & the Spectrum display with 100kHz AF as this gave clear sideband signals (& distortion products in course) & it was easy to select either upper of low sideband with ±100kHz off set of the RF signal generator.


See also my tech buls on "Spectrum Harmonic Demo circuit", "Scope RF Trick", "Tuned Circuit Scope Demo" & "Oscilloscopes".


Why don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


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