As everybody has already said, the LM139 isn't suitable for bandwidths above 10MHz.
Comparators are usually built with several stages of gain and no frequency compensation, unlike op amps - so their gain usually stays flat out to quite high frequencies, then falls off rapidly when each of the gain stages starts running out of gain at the same frequency.
Comparator data sheets rarely specify this frequency, but the data sheet for Linear Technology "ultrafast" LT10106, which has a 10nsec propagation delay, includes an application circuit which uses it to make a crystal oscillator - specifically a 10MHz to 25MHz crystal oscillator, which suggests that it isn't much use over 25MHz.
The application notes also includes the claim that the LT1016 has a gain bandwidth product of about 50GHz, which - for a part that has a typical gain of 3000, implies a bandwidth of 17MHz.
There are faster parts around - the Honeywell, then SPT and now Fairchild SPT9689 has its 3dB point at 900MHz, but it costs $20 and Arrow Electronics list a two week delivery time.
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Maxim have parts that are cheaper, and tolerably fast
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which do seem to be able to cope with 100MHz signals - the data sheet examples are of 100MHz square waves. Newark have 175 MAX9691ESA in stock for $6.00 each.
------------ Bill Sloman, Nijmegen