Hello Everyone. I have an exciter board on my desk with an MPX flatness problem. The range below 150Hz is 2dB over the rest of the spectrum. The device does not have a schematic and is made in smd, so intervention on it is not an option. I tried to correct the thing with a high pass filter, but then of course I have a problem with phase distortion. Square signal looks bad. Has anyone dealt with this problem and is there any scheme of phase corrector. I know that the stereo tool and breakaway have an RC corrector that works fine, so I'm interested in similar hardware builds.

Thanks!

You need add some capacitance .. or change varicap diode... do you heave good photo ? VCO part

It sounds to be a problem related to the PLL loop time constant. The PLL is trying to "correct" the modulation because the loop constant is not long enough, and it will get worse with the lower modulating frequencies. Either the exciter was not designed properly, or there is fault somewhere affecting the loop constant. I would start by checking capacitors in the PLL loop path to the VCO, its also possible its a dual speed PLL loop design - designed for a fast lock up, then switches to a low loop constant after lock. As a quick fix without diving deep into the workings of it, you could probably get away with adding a series resistor and capacitor between the PLL control loop and ground - I would start off with trying a 10K resistor and 220uF capacitor. However this will certainly affect the lock time and you may well be waiting over a minute for it to lock, however if that is the case then it will also sort out the modulation problem and you'll find modulating a 30Hz square wave should give linear and level modulation compared with every other modulating frequency. Also how are you generating the test tones? Do bare in mind that most sound cards on PC's are AC coupled and so you'll get tilt when generating a square wave, you're best to use a signal generator if you're not already.

Guys thanks for the tips. I think the problem is with the design of the device. I'll try to do a couple of experiments as you advised.
Unfortunately I do not have a signal generator. I use the M-Audio Air 192 semi-professional card and PC tools. The results on factory devices like RVR links were excellent. Flatness is up to 60KHz (sweep test), square at 30 and 60Hz looks as it should.