Re: HP Agilent 8753ES VNA repair [Help]
开云体育Hi, ? Starting point VCXO are correct frequency with 40.000MHz. At TP20 you have a frequency of 39.100MHz, and at the output of divider 40 you get 977.5kHz. This indicates, that the divider by 40 is correct but the frequency is wrong from VCO. ? At output of mixer 40MHZ from VCXO is mixed with 39.100 MHz (>16 MHz?) to have around 900kHz (909kHz). At output of mixer 40MHZ from VCXO is mixed with 39.280 MHz (< 16 MHz) you will have around 720kHz. Mixer is OK, but bad frequency from VCO, and good frequency from VCXO. ? Phase detector received bad feedback frequency from VCO (example for HIGH band, 909kHz instead 160kHz)… The PLL is open loop for a reason unknown at this time. ? Correction: TP6 is a TP from output mixer (H160kHz, L=4kHz), and TP7 is the reference for PLL (40MHz from VCXO divided by 250 for HIGH BAND and divided by 10,000 to have 4kHz). ? First question: Is the reference coming from the VCXO divided to have 160kHz for the high band and 4kHz for the low band correct at TP7? The VCO is functional, but the frequencies are incorrect, can the VCO range cover the expected frequency band, between 39.84 and 39.996 MHz. The electrical schematic shows the PLL section of an 8753D, which should be quite similar if not identical for the 8753ES. A way to test the VCO range should be found…remove serial resistor near the TP to inject DC voltage. But this verification can wait, and the verification of the phase detector and especially the integrator are the next points. ? I must go, come back later today. ? De?: [email protected] <[email protected]> De la part de MattSNE via groups.io ? Hello Yves, ? Thank you for your last post.? ? My VCO frequencies in TP20 are not correct. I have : - a RF signal is at 39.100 MHz, -1.3 dBm instead of 39.840 on your board for frequency >16 MHz? - a RF signal is at 39.280 MHz, -1.3 dBm ?for frequency <16 MHz? ? The 2sd LO is at 977 instead of 996 this is why my sampler FI output is at 23 MHz instead of 4 KHz. ? My VCXO is at 40.000 precisely. The VCO output to the mixer do not correctly converts the signal to L=4kHz and H=160kHz from what I measure. I have a signal at 909 KHz here. _ ? ? ? ? ? On PIN 2 on the divider output : 39.100 MHz divide by 40 : 977 KHz. -> The 2sd LO frequency I measure. ? ? ? ._,_._,_ 开云体育 Links: You receive all messages sent to this group. View/Reply Online (#150591) | Reply to Group | Reply to Sender | Mute This Topic | New Topic _._,_._,_ |