SLIM-PDM rev C
Phase Detector Module

Updated 10-21-08:  Update to reflect SLIM-PDM Revision B
Updated 2-9-09:  Update to add Schematic Error Note
Updated 8-11-09:  Update to explain voltage source option. Add revision history.
Updated 11-20-10:  Update to reflect SLIM-PDM Revision C, addition of 2 capacitors.
Updated 11-25-11:  Add links to Archived documents

SLIM-PDM, Phase Detector Module, size-A
Use your mouse's "right click" and "Save Link" to download:
a.  SKSLIM-PDM rev C, Schematic, in ExpressPCB software.
b.  LAYSLIM-PDM rev C, Parts Layout, in ExpressPCB software. For parts locations.
c.  PLSLIM-PDM rev C, Parts List in text format.  Open with any text reader.
d.  PWB-PDM rev A, PWB Artwork, in ExpressPCB software. Use to order from Express.
This SLIM-PDM is constructed on a common printed wiring board, the PWB-PDM (rev A).  Previously, there was a separate web page for the PWB.  I have deleted that page and combined the information on this page.  If you have constructed the SLIM-PDM rev A, using the PWB-PDM (rev 0) board, and would like to improve your PDM accuracy, see the Modification Procedure on the end of this page.

    The SLIM-PDM is a 360 degree, Phase to Voltage Converter.  It is specifically designed to operate at 10.7 MHz, but will operate in the KHz range up to 30 MHz.  The J1 and J2 inputs can be sine or square wave .  The min / max sine input is -20 dBm to +18 dBm.  The min / max square input is 10 mvpp to 5vpp.  Input impedance is nominally 50 ohms, but can be changed to any input impedance.
    The output, at J3, is a DC voltage that is proportional to the differential phase of the input signals at J1 and J2.  The 0v to +5v output should be loaded with 100 K or higher.  A realistic phase range is from 20 degrees to 340 degrees with error less than .1 degrees.
    Power requirements for the SLIM-PDM is +7 volts to +15 volts at 50 ma.

Option when using PDM in MSA/VNA
    The PDM has a provision to source +5 volts to a seperate Analog to Digital Converter module, the SLIM-ADC.  This allows both modules to use the same regulated +5volts.  This allows for more accurate Phase measurements.  The SLIM-PDM Rev 0 requires no change or modification for this application.  The SLIM-PDM Rev A requires the additional placement of FBx.

Engineering Change Notice, ECN-11-20-2010
    The PDM rev B may have a tendency to "self oscillate". This change is made to prevent this occurance, although this occurance is quite rare. If your PDM is operating normally, without any "strange" occurrances, you may omit the modification. To modify and update SLIM-PDM-revB into SLIM-PDM-revC, the addition of two capacitors is necessary (C24 and C25). See schematic, SKSLIM-PDM Rev C. Also see LAYSLIM-PDM-Rev C.

Revision History
Original Release: Revised 8-13-07
SLIM-PDM Rev 0, SKPWB-PDM Rev 0, PWB-PDM Rev 0, PLSLIM-PDM Rev 0

Revision A: Revised 4-30-08
SLIM-PDM Rev A, SKSLIM-PDM Rev A, PWB-PDM Rev 0, PLSLIM-PDM Rev A
Delete input filter ckt. Change Schematic and Parts List

Schematic Error Note:
  Updated 2-9-09:    In Rev 0 and Rev A versions of the PDM Schematic, there is an error in the drawing.  U4 has pins 1 and 2 interchanged.  The PWB is correct and if it was constructed per it's Layout, there is no problem.  Just annotate the Schematic to verify that the P1 connector, pin 3 is connected to U4, pin 2.  Annotate that U3 pin 4 is connected to U4 pin 1.

Revision B: Revised 10-21-08
SLIM-PDM Rev B, SKSLIM-PDM Rev B, PWB-PDM Rev A, PLSLIM-PDM Rev B
  Updated 10-21-08:  The SLIM-PDM-revA was redesigned into revB, to improve phase performance from about 1 degree resolution, to about .1 degree resolution.  The differences between RevB and RevA is the addition of a "one-shot", separation of the 5v distribution, and deletion of input filtering options.  The PWB has been revised for this change.  At the bottom of this page is information on how to modify the SLIM-PDM-rev A to improve it's phase performance.

Revision C: Revised 11-20-10 (current revision)
SLIM-PDM Rev C, SKSLIM-PDM Rev C, PWB-PDM Rev A, PLSLIM-PDM Rev C
  Updated 10-21-08:  The SLIM-PDM-revB was modified into revC, to improve the stability of the one-shots (U8 and U9). Two additional capacitors, C24 and C25 (22 pfd) are added to the schematic and added to the SLIM assembly. A new PWB is not released. PWB-PDM Rev A is still current. The layout is modified to LAYSLIM-PDM-revC. The schematic is modified from SKSLIM-PDM Rev B to SKSLIM-PDM Rev C.

Layout and parts locator for SLIM-PDM
slim/layslim_pdm.gif  slim/pwb_pdm.gif
    The document "layslim_pdm.pcb" is used to locate parts on the board.  Do not use to order PWB from Express.  Use pwb-pdm.pcb
    Note: The layout and schematic show that there are two ferrite beads supplying U5, FBx and FB2.  The installation of these beads is determined by how the SLIM-PDM is used in a final configuration of a high level system.
Updated 10-21-08:  The Layout (above) and schematic (below) show the addition of C24 and C25 to reduce the possibility of PDM instability.

SKSLIM-PDM, Schematic of SLIM-PDM
slim/skslim_pdm.gif
    Description of Option:  If the PDM is used as an independent module (not in a system), install FBx and delete FB2.  If the PDM is used to source its regulated +5 volts to another module, install both FBx and FB2.  If an external +5 volts is used to supply the entire PDM, install both FBx and FB2, and delete U1 (this option is not recommended).  If an external +5 volts is used to supply only the final stage of the PDM, delete FBx and install FB2.

Description of the Phase Detector Circuit
    The J1 (Reference) signal is buffered and squared by U2 and U3, and then applied to the input of U4.  U4 can be externally commanded to output the signal normally, or inverted (180 degree phase change).  U4's output signal (positive going, leading edge) triggers U9 to change it's output state to "1".  It will immediately reset to "0", creating about a 10 nsec, "one shot".   U9's output (positive going, leading edge), triggers U5 to change it's output state to "0".  U5's output will remain in a stable state of "0" until it recieves a "clear" signal on it's pin 6.
    The J2 (Input) signal is buffered and squared by U6 and U7.  U7's output signal (positive going, leading edge) triggers U8 to change it's output state to "0".  It will immediately reset to "1", creating about a 10 nsec, "one shot".   U8's output (negative going, leading edge), clears U5 to guarantee it's output state at "1".  U5's output will remain in a stable state of "1" until it recieves the next "clock" signal on it's pin 1.
    Note: U2 and U6 are low current, self biased buffer amplifiers, to allow either sine or square inputs.  Do not substitute these components without design consideration of power consumption.
    The output of U5 is a series of square waves with a duty cycle dependent on the amount of time that U5 is in a "0" state or in a "1" state.  This duty cycle represents the ratio of the phase of it's two input signals.  The U5 output is coupled to a resistor and capacitor, which filters out the high frequency square waves.  This leaves an integrated DC voltage that will vary from 0 volts to + 5 volts, depending on the phase difference of the two input signals.  This DC signal is called Phase Volts, at J3.  The integration filter is not low impedance, due to the 10 K ohm series integration resistor.  Therefore, any load connected to J3 must be high impedance to prevent excessive loading.  Loading should be 100 K ohms or greater.  The input resistance of 1 Meg ohms, typical of digital voltmeters or Analog to Digital Converters is the expected load for the PDM.

Uncertainties and Unlinearities of the SLIM-PDM (The Dead Zone)
    If this were a "perfect" Phase Detector, and the two input signals were in phase by 0 degrees, the output voltage would be 0 volts.  If the two signals were out of phase by 360 degrees, the output voltage would be +5.0 volts.  Since there is no difference between 0 degrees and 360 degrees, a Phase Detector would be "confused", and it's output would not "know" whether to output 0 volts or +5.0 volts.  This is an "indeterminate" phase detection area and the output will be very noisy.  Also, this phase detector has an area that is unlinear, due to several factors.  The time durations of the two pulses controlling U5, the internal speed of U5, and the rise/fall times of the input signals and output signal.  This area of indeterminance and unlinearity is simply called the "dead zone".  It occurs from about -20 degrees to about +20 degrees from when the two input signals are in phase (0/360).
  This is the reason for U4, to invert the Reference input signal and change it's phase by 180 degrees when the phase detector approaches, or is in the "dead zone".  This phase inversion of 180 degrees can be subtracted from the final measurement results, if U4 has been commanded to "invert".  However, due to the internal speed of U4, this will not be exactly 180 degrees.  It will be quite close, but it needs to be measured and substituted for the theoritical 180 degree phase inversion.

Converting Phase to Voltage
    Assume, for the moment, that this Phase Detector is "perfect".  That is, no "dead zone".  It has a full dynamic range of 360 degrees.  If the Reference signal is the only signal entering the phase detector, the U5 output will remain in a "0" state.  When the Input signal is applied, U5 will be forced to change it's output state to "1" and it will remain there until the next reference pulse.
    Now, let us assume the Input signal lags the Reference signal by 1 degree.  The Reference signal triggers U5 into the "0" state.  Then, after 1 degree, the Input signal forces U5 into the "1" state.  It remains in the "1" state until the next Reference pulse.  And, this will not occur until 359 degrees later.  Therefore, the output of the U5 will be a square wave with a duty cycle (low for 1/360 of the time and high for 359/360 of the time).  The integrated DC Phase Volts will be equal to (360-1)/360 x 5.0v, or 359/360 x 5v.  This would be 4.98611 volts.
     Now, assume the Input signal lags the Reference signal by 10 degrees.  The Reference signal triggers U5 into the "0" state.  Then, after 10 degrees, the Input signal forces U5 into the "1" state.  It remains in the "1" state until the next Reference pulse, which occurs 350 degrees later.  The output of the U5 now has a duty cyle, of low for 10/360 of the time and high for 350/360 of the time.  The integrated DC Phase Volts will be equal to (360-10)/360 x 5.0v.  This would be 4.8611 volts.
    For both cases, the output voltage is proportional to the amount of time the output of U5 is high compared to the total period.  The following computations are used to determine the output voltage:
    If Input lags Reference by 1 degrees, then (360-1)/360 x 5.0v = 4.98611 volts
    If Input lags Reference by 10 degrees, then (360-10)/360 x 5.0v = 4.8611 volts
    If Input lags Reference by 45 degrees, then (360-45)/360 x 5.0v = 4.375 volts
    If Input lags Reference by 72 degrees, then (360-72)/360 x 5.0v = 4.000 volts
    If Input lags Reference by 90 degrees, then (360-90)/360 x 5.0v = 3.750 volts
    If Input lags Reference by 180 degrees, then (360-180)/360 x 5.0v = 2.500 volts
    If Input lags Reference by 270 degrees, then (360-270)/360 x 5.0v = 1.250 volts
    If Input lags Reference by 288 degrees, then (360-288)/360 x 5.0v = 1.000 volts
    If Input lags Reference by 315 degrees, then (360-315)/360 x 5.0v = 0.625 volts
    If Input lags Reference by 350 degrees, then (360-350)/360 x 5.0v = 0.1389 volts
    If Input lags Reference by 359 degrees, then (360-359)/360 x 5.0v = 0.01389 volts 
Since the SLIM-PDM is not perfect, and has a "dead zone", data taken below 45 degrees and above 315 degrees is subject to greater error than the range between 45 and  315 degrees.
    The calculations, above, assumed that the voltage applied to the phase detector (U5) is exactly +5.0 volts.  This will never be the case, and is not important.  In the formulas, substitue the 5.0v with the actual voltage applied to U5 pin 8.  In the SLIM-PDM, it will be between +4.90 and +5.10 volts, due to the tolerance of the voltage regulator (U1), or the external reference applied at P2.

Using the Phase Detector Module
    The SLIM-PDM can be used as a "stand-alone" module for phase to voltage conversion.  If so, it must be calibrated for the use it is intended.  The user should be aware that sine wave inputs must be amplitude stable for accurate results.  Otherwise, AM to PM conversion error is introduced by the buffer amplifiers (U2 and U6), although minimal.  Temperature changes will also effect the accuracy.
    When the SLIM-PDM is integrated into a higher level system, the same conditions exist.  I designed this SLIM-PDM specifically to be integrated into a Vector Network Analyzer.  The SLIM-PDM's voltage output is directly coupled to a Voltage to Digital converter, which creates a Phase to Digital Converter, bypassing the voltage conversion algorithims.  The conditional phase errors of the PDM are calibrated at a system level, rather than at a module level.

Updated 10-21-08  Modification of the SLIM-PDM-revA
    The following modifications can be performed to increase the phase measurement accuracy of the SLIM-PDM-revA.  If the builder is satisfied with phase measurements that are accurate to +/- 1 degree, the MODS 1-4 need not be done.  These mods will increase the accuracy of the PDM to about .1 degrees and the total accuracy of the VNA system to about +/- .2 degrees.

Pictures of SLIM-PDM-revA and modifications to increase performance.
  slim/pdm.JPG  slim/modpdm.jpg
    The first photo is the SLIM-PDM-revA without modifications.  It uses the Rev 0 version of the PWB.  The second photo shows where to cut the traces and add components.

MOD 1: Fix phase error caused by Fairchild '74 flip-flop.  The internal timing of U5 is affected if it's clock signal at it's pin 1 is at a high level when it is "cleared" at Pin 6.
a. Cut trace between U4-4 and U5-1, see "x" in photo (on top of capacitor)
b. Install a 10 pfd capacitor in series between U4-4 and U5-1 (the cap with "x")
c. Install a 1.0 K ohm resistor from U5-1 to ground at C9.
This modification creates a "spike" for triggering U5.  This is an easier modification than adding another "one-shot".

MOD 2: To decrease circuit cross-talk, via the common power traces. This separates the common 5v feed from U1 to individual feeds.
a. Cut the +5volt power trace between the junctions of R3, R4, R6, R8.
  The trace between R3 and R4 is kept intact and the trace between R6 and R8 is kept intact. The short trace length between these two junction paths is what is cut.  see "X" in photo
b. Add a jumper wire (22 to 26 ga.) from C5 (+5v) to the junction path connecting R6 and R8. (--5v)
This modification helps isolate the Limiter squaring circuit from the Reference squaring circuit.

MOD 3: Increase linearity of Reference buffer circuit, U2 and U3.
a.  Remove R2.  Install R2 (on it's side) on U3-4.  Add a jumper wire from the open end of R2 to the junction trace of C12 and R1 (where R2 was removed).
b. Cut +5volt power trace between the junction of R3 and C7. see "X"
c. Add a .1 ufd capacitor from R3 (U3-5) to the ground at C10.
d. Install a 10 ohm chip resistor at the +5v junction of C7. Stand it on end.  Attach a jumper wire from the open end of the 10 ohm resistor to C5 (+5v).  see "5v ------"
This modification isolates U2 from U3, which makes the buffering action of U2 more linear.

MOD 4: Allows the Phase Detector IC (U5) to be powered by the SLIM-ADC module.  This modification is for better phase accuracy in the VNA system.  It requires modifications in two modules, the SLIM-PDM and the SLIM-ADC

I.  Modification to SLIM-PDM:
a. Install FB1 and C19. Installing P2 is optional.
b. Cut 5 volt power trace between C5 and ferrite bead, FB1.  see "X"
c. Install a 10 uf capacitor between FB1 and the ground at U5-4

II.  Modification to SLIM-ADC-16 (or SLIM-ADC-12):
d. If an external wire is connected between the SLIM-PDM-P2-2 and the
  SLIM-ADC-P1-2, remove it. (corrected error on 11-22-09, was SLIM-PDM-P2-1)
e. Install U1 (remove jumper under U1 if installed)
f. Drill a hole (appx 0.020 inch dia) next to U1-1. Trim away ground
  plane on bottom side to prevent shorting.
g. Insert a wire (22 to 26 ga.) into hole from bottom.
    Bend end of wire over the +5v trace at U1-1, and solder.
    Connect other end of wire to SLIM-PDM-P2-2. (corrected error on 11-22-09, was SLIM-PDM-P2-1)
Now, the SLIM-ADC-16 (-12) will supply +5v power to phase detector
  flip-flop (U5) in the SLIM-PDM.  (appx. 1.6 ma)