Instructions for powering up and providing the necessary inputs to the PAF Digital Downconverter
A single blade of the PAF Digital Downconverter comprises 8 independent receiver channels. Below is a
photograph of a single blade as seen from the "front." Oriented as shown, the RF coaxial inputs are on the left, the
fiber outputs are on the right, and the bias, M&C, and LO are at the top.
Location of Inputs and Outputs:
There is a single Micro-D connector at the top with 3 power pins for bias and 7 control pins for setting the internal
attenuators and reading out the monitor points via an SPI bus. As this is a somewhat specialized connector, a test
cable will be supplied with these lines labeled (part no. A104082-001 from Omnetics. For the final instrument on the
telescope, I would recommend a shielded version with EMI backshell.)
The main RF inputs are the eight SMA connectors on the left side. There are also 8 SMP (press-in) connectors in
between these (not visible in the above photo), which are intended as calibration inputs. As shipped, these will be
plugged with gold-colored dust-covers. It is recommended that these dust plugs be left in place when the calibration
ports are unused. Also, at the very bottom of the left edge is a ninth SMA input, offset from the main row, which is
the sample clock input (black cable coming in from the left in the above photo).
At the top of the module next to the Micro-D connector are two more SMA launchers, one for LO input, and the
other for LO output (these are labeled). When several blades are used, the LO is daisy-chained from one to the next.
For testing a single blade, the LO output should be terminated.
Nominal signal levels:
+5V bias -- This supplies the analog circuitry and M&C functions. The expected current draw is about 2.7A.
+3.5V bias -- This supplies the digital and photonic circuitry. The expected current draw on this supply is about
6.8A. Be aware, however, that there is an internal sequencer that will delay the full current draw for about a second.
A small fan is required to keep the module from overheating when this supply is used. The chassis should get warm
to the touch, but not hot. (Feels roughly like body temperature.) NOTE: this supply is not required to test the M&C
functions.
GND -- This is the main ground return. It is connected to the chassis. Note that with both positive supplies turned
on, it will draw almost 10A of current.
LO input -- Supply a local oscillator tone nominally at 1450 MHz with +5 dBm power. The LO output port should
be terminated.
RF input -- For testing, a laboratory L-Band (1.2-1.7 GHz) preamp with about 30 dB of gain (not critical) should be
connected to this input. This is essentially a stand-in for the cryogenic gain stage on the PAF, and brings the noise
level up to a point that it can be digitized. The input to this amp may be terminated, or it may be connected to a
synthesizer for providing CW diagnostic inputs. If a synthesizer is used, please put about 90 dB of attenuation
between it and the preamp to avoid over-driving the receiver. (I use a 60 dB and 30 dB pad, with the synthesizer set
to 0 dBm output.)
Sampler clock input -- This is the SMA connector at the end of the row of RF inputs (not to be confused with the
serial SPI clock or "SCLK" line on the Micro-D connector.) Supply a 155.52 MHz tone with about 300 mV
amplitude, or approximately 1 mW of power (not critical). NOTE: This signal is not required to test the M&C
functions.
IF diagnostic outputs -- These are the 16 SMP connectors on the right side of the module (not visible in the
photograph). These are 10 dB-coupled versions of the I and Q analog outputs for each channel just before the
samplers. The module will be delivered with gold-colored dust plugs on these outputs. Pull the dust plugs straight
out with a pair of pliers (my fingers are not strong enough!) To minimize self-interference, these should be replaced
before operating on the telescope (I find a #10 socket-head machine screw is the perfect insertion tool.) If SMP
cables are not readily available, I would recommend part no. SCA72086-xx from Fairview Microwave. The IF band
is low-pass, from DC to about 75 MHz, and with the above inputs, it should be visible on a laboratory spectrum
analyzer. With these test points, it should be possible to verify all M&C functions, including attenuator settings,
without having to power up the digital circuitry or supply the sampler clock, as noted above, nor having to connect
to the optical fibers.
Nominal monitor point readings:
There are 8 RF power monitor points and 16 IF power monitor points. The bus protocol for accessing these and the
appropriate software scaling factors were communicated separately from this document. Their actual values will
depend somewhat on the gain and noise figure of the laboratory preamp used, as well as the setting of the
attenuators, but in our setup the RF power typically reads as -45 dBm and the I and Q outputs read approximately 28 dBm when the attenuators are set to mid-range. Note that the attenuators are in the RF path following the monitor
point, so changing the attenuator values will affect the I/Q monitors, but not the RF monitor.
Word of caution: Permanent output fiber connectors have not been spliced on yet. The ends have been stripped to
allow use of a bare-fiber adapter for test and calibration purposes. They are very delicate and one should avoid
handling them. It is not a very big deal if the stripped ends break off, however I would prefer not to shorten them
any more than necessary. As extra incentive, keep in mind that one could hardly engineer a more perfect splinter
than a short rod of glass 125 m in diameter!