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Group delay in VHF FM

S

Studio1

A colleague was discussing radio theory the other day and presented me with a question about 'group delay' with FM signals.
I wasn't able to give him a satisfactory answer.
The application he was relating to was a simple Fm transmitter running at 88 MHz with a cavity filter on the output and how this affected the sidebands/pilot phase etc.
Some thoughts on this issue would be useful.
 
Studio1 said:
A colleague was discussing radio theory the other day and presented me with a question about 'group delay' with FM signals.
I wasn't able to give him a satisfactory answer.
The application he was relating to was a simple Fm transmitter running at 88 MHz with a cavity filter on the output and how this affected the sidebands/pilot phase etc.
Some thoughts on this issue would be useful.
Click to expand...
Been a long time since I broached this topic, but it "seems" that minimum group delay and maximum bandwidth go hand in hand. The cavity filter will reduce the bandwidth and probably have a detrimental effect on the group delay.
 
Thanks Bob, that's exactly what I was thinking too... and we sort of talked along these lines.

His questions to me were spurned on by a change in antenna. He said that the dipole antenna sounded 'thin', but when
he changed over to the circular polarised antenna there was a definite improvement in the sound quality.

Subsequently we started talking about the cavity filter and group delay and that's a subject I really know nothing about!
 
Essentially, in a reactive system, propogation speed is affected by frequency. Somewhere I have it in mind from a conversation with a guru, in an FM system, if the difference in delay highest frequency of interest to the lowest theough the system is less than about 50 nS, you won't hear or measure the effect. In one multistation system I ran, ERI added what they called 'skirt filters' at one station's input to improve group delay through the system. I suspect this was the same thing that we used to do with analog (gasp!) multiplexed teletype (gasp again!) tone packages which were multiplexed onto microwave systems. What you did was measured the end- to - end delay of the tones, and then added elay to the fast ones such that everything came out the other in at the same time.
 
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