Radio Intermediate Frequencies

A fact which sometimes confronts those who endeavour to bullshit the bullshitters. ;-P

Cheers! Rich (OBTW, there's no apostrophe in ordinary plurals, like 'guys'.)

"James Douglas" wrote in message news:7sudnesml_IGy27enZ2dnUVZ snipped-for-privacy@comcast.com...

Question comes up quite often and I must admit it's surprisingly awkward putting some representative bits together without throwing the baby out with the bathwater. (fun though!). The maths is minor. It's the visualisation and implemenation where the art and complexity resides. This circuit is about as simple as it gets and should go into Circuitmaker OK. I've also stuck on a Switchercad listing of the same thing.

___ IN914 .--------|___|-o-->|---o--------o---------o | 1k | | CC | | | 2Vpp | | .-. .-. /+\\ 1kHz DD| | | | | | ( ) | | | | | | \\-/ | | '-'1M '-'1M | _1k | .-. AA | BB| | .-|___|- | | .-o-. .-o--. | | | | | | | | | /+\\ '-' C| | C| | | ( )4Vpp 100k| C| --- C| --- | \\-/ 4kHz | C| --- C| --- | | | | |28nF | |10nF o------o---------------o------o---o-----o----o-- 0V 100mH 100mH

(created by AACircuit v1.28 beta 10/06/04

Point of interest is that the waveform at "DD" is actually a true 'mixing' of the 2 sinewaves. The voltage waveform may look distinctly interesting and 'modulated' but it isn't, it's still just two boring added sinewaves. The point at "CC" (after the diode rectifier) is the needed true 'multiplication' of the 2 sinewaves, i.e. they really have been modulated. Notice also that the waveform at "CC" looks nothing remotely like the 2, neat, sum and difference frequencies the books talk about. The critical point is that the "CC" diode created, massively distorted undulating waveform, can now be broken down into -numerous- individual sinewaves. Two particularly strong and useful ones, being a sum frequency of

5kHz and a difference frequency of 3kHz. (each about 90mVac). There's also even bigger amounts of the original 1kHz and 4kHz in there, along with shedloads of other rubbish and is the reason for the added the tuned circuits. Theses are sharply resonant at 3kHz ("AA") and 5kHz ("BB") and will for demo' purposes act as a simple spectrum analyser by cleanly picking out these particular frequencies from the general bedlam, just the same as a radio 'I.F. strip' would. If Circuitmaker has an 'FFT' option then the tuned filters are of no consequence. I'd also add, that the 2 input sinewaves don't have to be sines or those particular voltages. They were just picked for clarity and in reality can be any old waveshape. john

(Switchercad *.asc drawing follows. Do a '.TRAN' of about 0.3secs)...

Version 4 SHEET 1 880 680 WIRE 0 208 0 32 WIRE 0 336 0 288 WIRE 96 208 96 160 WIRE 96 336 96 288 WIRE 144 32 0 32 WIRE 144 160 96 160 WIRE 240 32 224 32 WIRE 240 160 224 160 WIRE 240 160 240 32 WIRE 272 32 240 32 WIRE 384 32 336 32 WIRE 384 112 384 32 WIRE 384 320 384 192 WIRE 496 192 496 176 WIRE 496 288 496 272 WIRE 528 32 384 32 WIRE 528 64 528 32 WIRE 528 176 496 176 WIRE 528 176 528 144 WIRE 528 288 496 288 WIRE 528 320 528 288 WIRE 560 176 528 176 WIRE 560 192 560 176 WIRE 560 288 528 288 WIRE 560 288 560 256 WIRE 640 192 640 176 WIRE 640 288 640 272 WIRE 672 32 528 32 WIRE 672 64 672 32 WIRE 672 176 640 176 WIRE 672 176 672 144 WIRE 672 288 640 288 WIRE 672 320 672 288 WIRE 704 176 672 176 WIRE 704 192 704 176 WIRE 704 288 672 288 WIRE 704 288 704 256 FLAG 0 336 0 FLAG 96 336 0 FLAG 528 320 0 FLAG 672 320 0 FLAG 384 320 0 SYMBOL voltage 0 192 R0 WINDOW 3 -84 122 Left 0 WINDOW 123 -85 152 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value SINE(0 1000m 1000) SYMBOL voltage 96 192 R0 WINDOW 3 -18 122 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value SINE(0 2000m 4000) SYMBOL diode 272 48 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D1 SYMATTR Value 1N914 SYMBOL res 240 16 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 1k SYMBOL res 240 144 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R3 SYMATTR Value 1k SYMBOL ind 480 176 R0 SYMATTR InstName L2 SYMATTR Value 100m SYMBOL cap 544 192 R0 SYMATTR InstName C2 SYMATTR Value 28000pF SYMBOL res 544 160 R180 WINDOW 0 36 76 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName R1 SYMATTR Value 1meg SYMBOL ind 624 176 R0 SYMATTR InstName L1 SYMATTR Value 100m SYMBOL cap 688 192 R0 SYMATTR InstName C1 SYMATTR Value 10000pF SYMBOL res 688 160 R180 WINDOW 0 36 76 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName R4 SYMATTR Value 1meg SYMBOL res 368 96 R0 SYMATTR InstName R5 SYMATTR Value 10k TEXT -90 26 Left 0 !.tran .3 TEXT 368 8 Left 0 ;CC TEXT 488 160 Left 0 ;AA TEXT 632 160 Left 0 ;BB TEXT 224 8 Left 0 ;DD

Note that an ideal multiplier would output only the sum and difference frequencies, not the originals. But it is much simpler to simply build a non-linear amplifier that has strong distortion components (quadratic distortion) at the sum and difference frequencies, and it is OK since everything but the diference is filtered out by the IF stage.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Home of DaqGen, the FREEWARE signal generator

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: I believe a Direct Conversion design, which eliminates the need for an IF, : makes separating the modulation from the RF signal quite simple. Direct : Conversion uses demodulation rather than envelope detection, so the signal : is baseband audio straight out of the mixer. A simple low pass filter (RC) : will prevent RF from affecting the following audio stage(s). If one wishes, : wideband and narrowband audio filters can be switched in/out for "music" and : CW.

Don,

While direct conversion has the advantages that you mention, it also has some serious disadvantages. One of the biggest disadvantages is that Direct Conversion Receivers suffer from large DC offsets caused by self-mixing of the LO (i.e. LO couples into RF signal.) Most of the solutions to this problem are not very elegant -- they involve calibration/trimming/etc. It's also difficult to prevent the coupling of the LO into the input signal, because of the way the mixers, themselves are typically designed (I've only worked with CMOS mixers -- this may not apply as much to Bipolar designs.)

Joe

The problem you describe is certainly possible with a poor design, such as using a single diode mixer and not using an isolation stage between the signal input and the mixer. But these are avoidable.

More importantly, the OP asked about methods (I assumed modern ones) that do not use an IF. Direct Conversion can easily provide good results, just as a superhet can provide good results *if properly designed*.

Don

I am glad I checked back on my post T H A N K S. I also recently downloaded still another simulation package, although not as easy to use as CircuitMaker yet it's seem Ok,

Anybody using B2 SPICE v5?

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