can i use a function generator in place of a rf signal generator

[croyleje]

hi everyone,
would using a 10MHz function generator with a sine wave work to align an IF filter rather then a RF signal generator?  i don't have access to a RF signal generator and would like to try to complete this with out buying one any input from you guys would be greatly appreciated.
Thank you,
Jason

[saturation]

Yes, in most cases.  The alignment procedures for your IF stage should dictate what you need,  typically all it needs is accuracy of generator output and stability, you can always rig interface requirements to match the output impedance of the FG.  DDS or better FG can provide such stable output.

Analog FG and RFG are inaccurate and drifty by today's standards, so you'll need a frequency counter to confirm the output and see where it drifts to keep it on track for your needs.

[T3sl4co1l]

Yes but...  The tricky part is, an analog FG's output is not as stable (over any time scale*), nor as precisely adjustable, or as accurate in display.  You'll want a frequency counter to be sure you're aligning it to the correct range, as mentioned.

*Meaning, over short periods of time, noise in the circuit perturbs the apparent frequency, phase, amplitude, etc., effectively spreading the bandwidth.  It's usually good enough for LC tuned circuits, but tightly aligned e.g. ceramic or crystal filters may not be so forgiving.  Over longer periods, capacitance and resistance in the circuit drift, or vary with supply voltage, etc.  Weak effects, but it all matters at this level.

Digital FGs use a DDS, which offers good long-term stability and precise settings, but they achieve this at the cost of approximating that stable frequency by jumping around all the time.  This is partially helped by using a good filter to remove the internal clock rate, but lower frequency side bands, aliasing and spurs still remain.  The quality of a DDS is specified by the level of these spurs.  For aligning an IF, the spurs are probably not important, as long as you can be certain that the frequency you're aligning to is the fundamental of the signal.

Tim

[croyleje]

thank you both for your help and input i don't normally do many RF project but i am making a WWV 10MHz receiver i got all the schematics and parts now and got a copy of the alignment procedure which says to feed in a 10MHz signal from a RF signal generator.  i just wasn't sure if my function generator would be good enough.  i also have a very nice racal dana counter that i can use to check the output freq of my generator.  other then the obvious wave shapes, sweep and such is there any real difference between a function generator and signal generator.  from what i can find most places only say the frequency range is what separates a function generator from a signal generator.  also my function generator has an AM modulation feature so i was hoping i could go without buying a RF generator for these simple alignment jobs i will need to do for a few RF projects i have planned.
Jason

[nixfu]

RF Signal generators usually have larger frequency ranges (1-400Mhz+) instead of 1-20Mhz common with function generator.

RF Signal generators, can also generate more types of modulation such as FM, and they can adjust the BANDWIDTH of the modulation so you can test the impact of the width of the modulation on your design filters, demodulators, mixers etc.

But for testing a bandpass filter, as long as its less than 20Mhz then a simple signwave out of a function generator can usually do what you need to do.

[croyleje]

thanks nixfu i am pretty confident that my thinking was correct and my function generator has modulation options on it so i think ill be good but will keep my eyes open for an old RF sig gen.  thanks everyone for your help and information.
thank you,
jason