unlikely that one could discern a readout much beyond
0.5% (1/4 minor division). For that reason, the errors of
the 10% and 30% bands can almost be dismissed.
For examining DUTs which have very wide
bandwidths, the setup would be 100% sweep; simply start
at an RF band lower than necessary and run through the
RF bands beyond what is needed, pausing at various
responses in the slopes and expanding for closer
examination. The RF generator has excellent flatness of
output level, so band hopping will not affect the test. All in
all, these specs are satisfactory for most of the tests you
This sweep adapter was a nice fit for my RF generator
design. For readers that have commercial generators,
some changes are likely required. The basic circuit
architecture will remain the same but modifications will
have to be made for it to work correctly. You first have to
determine the varactor’s tuning range (Vt), which is usually
about one octave of frequency. To do this, you will have
to measure the Vt span applied to the varactor by
connecting a DMM directly to the varactor bias resistor
ON THE INPUT FEED SIDE as shown in Figure 4.
Ultimately, this may be the tie-in point from the adapter’s
Vt output lead.
The feed resistor will usually be in the 5-100K range.
Tune the generator through its full travel and make note of
the voltage and span read. Due to the wide range of
designs from different manufacturers, these bias voltages
can be all over the map. They may be anywhere from five
volts to 30 volts, and in a positive or negative direction, or
even bi-polar. Once you have established the starting
voltage and span, the
adapter Vt out lead has
to present that voltage to
your particular generator.
be as simple as an added
op-amp here to scale,
offset, and/or invert the
voltage that the adapter
currently outputs. A
better way (if your skills
are sufficient) would be
to change gains,
polarities, and inversions
in the unit itself, thereby
not having to add more
stages. The maximum Vt
spans that can be
obtained with the
adapter’s power supply
are from - 10. 5 volts to
+ 10. 5 volts. Beyond that,
you will need a separate
higher voltage power supply which would only be used in
the Vt output stage.
One word of caution here is that the Vt feed point
must be carefully chosen due to unwanted voltage
division of the internal and external Vts. It’s better if you
can locate a tie-in point farther upstream from the varactor
— especially a summing amp because there is no
interaction of inputs at their summing point. This is not
rocket science, but if undertaken, you should have a
certain knowledge and confidence of what is required
here. It goes without saying, a good factory service
manual or at least a schematic diagram is needed.
Before ending this article, I will again present a quick
rundown for the sweep setup:
• Sufficient warm-up time for both units.
• Generator-RF band of interest.
• Adapter — S5 Sweep on; S1a in Auto position; Vernier
control fully CCW.
• Generator — Set RF tuning controls for start frequency.
• Adapter — S3a in Set position; switch S4 for desired
span; advance Vernier CW to stop frequency.
• Place S3a in Run position. Start testing.
Note: Manual tuning may be switched in or out at any
given time for spot checking an exact frequency at any
point along the scope X axis trace. This will not interfere
with the setup already in place.
So, that’s it. I hope I have not left anything uncovered
here, but as I have done in previous articles, I’ve included
my email address for any problems that might arise:
firstname.lastname@example.org. Also, I will have additional information in
packet form including artwork, etc., available directly from
me and also at the article link. NV
38 February 2017
PART DESCRIPTION PART DESCRIPTION
R1 22K* C1 1,000 pF MLC
R2 1.0 M* C2 1.0 mfd (Do not use ceramic or
R3 20.5K* C3 22 mfd electrolytic
R4 39.2K* C4, C6 0.1 μf
R5 2K C5, C7 10 μf
R7 20K D1, D2 1N916 or equiv
R8 510Ω Q1 2N4401
R9 1.5M* Q2, Q3 2N3906
R11 100Ω IC1 TLO84
R12 3.3K IC2 TLO82
R13 1.21K* Optional detector diode BAT62-02
R15 6.98K* S1, S3 DPDT min toggle switch
R16 2K* S2 SPST (part of Sweep Rate pot)
R17 1.02K* S4 SP three-position wafer switch
R18 3.3K S5 SP min toggle switch
R20 2K Sweep Rate pot 5K linear w/sw
R21 12K Manual Sweep pot 10K/10T
R22 8.2K* Sweep Vernier pot 10K/10T
R23 12K* Manual Calibration 2K trimpot
R24 5.1M Ramp Calibration 2K trimpot
R25 51K DC Set Calibration 5K trimpot
Over-Range Calibration 5K trimpot
Note: All resistors
1/4 watt 5%
* 1% resistors