Intermediate Frequency choices

Sounds like an excellent explanation to me. What part don't you understand?

Cheers

Phil Hobbs

School boy question. Real men use a spur calculator ;-)

...Jim Thompson

Wow, you Arizona cowboys are really high-tech.

Cheers

Phil Hobbs

When I'm reviewing the answer as mentioned in the following, I couldn't figure out why, nor find the internet resources that would help explain why this is the case. It would be great to hear some comments here. Thanks in advance.

"A lower IF allows one to achieve higher selectivity for a given filter quality factor and thereby achieve greater selectivity (ability to reject adjacent signals).

A higher IF shifts the image frequency farther away from the frequency of the desired signal. This makes it more practical to use a pre- selection filter or preselector (inserted in front of the mixer?s RF input) to suppress any signals that appear at the image frequency."

Of course, which is why, in the old days, the dual-conversion superhet topology was popular for HF to UHF communication receivers. Improvements in manufacture of crystal, ceramic and SAW filters, helical resonators, etc., have provided the required form factors using single, moderately high, IFs.

Chris

Oh, no... Real men use dual conversion! Best of both worlds.

tand?

What helps is you need to view the communications system from the standpoint of a fixed channel bandwidth. You can move filter center frequencies all over the place, but the net response (i.e. cascade of filters as viewed from baseband) has to be fixed.

Once you lock the bandwidth, the filter arguments make more sense.

As long as the frequencies are far enough apart, i.e. you use some really enormous first IF--often well above f_RF--or the tuning range is very small. Otherwise you have way more spurs to worry about with double conversion.

Cheers

Phil Hobbs

Spoken like an IC guy. The rest of us have to adjust the tracking of all those tunable filters, which clarifies the issues very fast!

Cheers

Phil Hobbs

My confusions about the explanations are how lower IF actually allows greater selectivity, and how higher IF actually shifts the image frequency farther away from the frequency of the desired signal. Hopefully my questions are clear enough in terms of not being able to figure out at the moment why these are the case.

Thanks.

tand?

Immediately squaring the spur problem unless you are REALLY careful.

Jim

Usenet netiquette: Don't top-post. It makes the thread hard to read.

Do you know what "image frequency" and "quality factor" (i.e. _Q_) are?

Do you know how to compute the IF and image frequencies, given the RF and LO?

Cheers

Phil Hobbs

You need to see the difference in frequency space.

George H.

Never owned one myself. But the Garmin people I worked with did.

...Jim Thompson

"ssylee"

I'm reviewing some notes and came across this question: "Why might one choose a low intermediate frequency (IF)? A high IF?"

When I'm reviewing the answer as mentioned in the following, I couldn't figure out why, nor find the internet resources that would help explain why this is the case. It would be great to hear some comments here. Thanks in advance.

"A lower IF allows one to achieve higher selectivity for a given filter quality factor and thereby achieve greater selectivity (ability to reject adjacent signals).

So a filter with 10 kHz bandwidth operating at 1 MHz has a Q of 100. But a similar filter, also with Q of 100, operating at 100kHz would have a bandwidth of 1kHz - so it is 10 times more "selective" that the 1MHz version.

The term "selectivity" = essentially the same as bandwidth in relation to radio signals.

A higher IF shifts the image frequency farther away from the frequency of the desired signal.

With a higher IF frequency, image frequencies are more distant from the RF ones and hence less troublesome to remove by filtering.

This makes it more practical to use a pre- selection filter or preselector (inserted in front of the mixer?s RF input) to suppress any signals that appear at the image frequency."

The majority of radios ( ie AM or FM broadcast receivers ) have a *variable* RF filter which is adjusted as you "tune" the radio to operate at the actual radio frequency to be received. This allows lower IF frequencies to be used where image frequencies may fall inside the RF range to be received.

..... Phil

Thank you Paul Allison for your explanations.

ne

ty

cy

RF

derstand?

=A0 =A0 =A0...Jim Thompson

The other thing engineers forget about is high side versus low side mixing. Low side mixing reverses the spectrum, while high side doesn't. Most text books show the spectrum as a box, so this problem isn't obvious. But when you compute sums and differences, the reversal falls out in the math. I think for FM it doesn't matter, but don't quote me. For sure sideband cares about the reversal.

For radio gear I've owned. I've always found the best performance is with really high frequency IF in the first stage. This is very true for military airband monitoring.

rstand?

It helps to draw a diagram. Frequency on the X axis. Energy or whatever you want to represent a signal level on the Y axis. It is useful to show the spectrum being translated as something other than a single frequency. Most text books show a box with endpoints for the frequency limits. The mixer frequency is a single frequency, often drawn as an arrow pointing upwards. Compute the sum and difference frequencies.

How did their horses feel afterwards?

Cheers

Phil Hobbs

understand?

For AM it doesn't matter at all, for FM _audio_ it doesn't either, since you can't hear a constant 180 degree phase shift, but for e.g. FSK, sideband inversion gets ugly--ones become zeros, etc. SSB gets mixed up too--I recall seeing a phone scrambler based on that idea once.

Cheers

Phil Hobbs