Another liquid-cooled GatesAir transmitter installation was completed for WYPR, Baltimore, MD. This unit replaced a Continental 816 which had a long life.
The area around the transmitter site is not the best neighborhood. The building was formerly the WBAL-TV transmitter site and was built circa 1947.
WYPR transmitter siteOriginal four-legged Blaw Knox self-supporting tower from the WBAL-TV daysPump station and HTF tubingWall-mounted heat exchangerFLX-20 transmitter
The transmitter room is a little tight, so it was difficult to get a good shot of the front of the unit.
The station is running HD Radio with -10 dBc.
This site has a strange 3-phase delta AC power configuration. The middle leg is at ground potential, and the other legs all measure 240 VAC to ground. I’ve never encountered that before. This is known as a corner grounded Delta, which gets rid of the high leg associated with most closed 3-phase delta systems.
Ultimately, all the leg-to-leg measurements are 240 volts, so the power supplies are satisfied. With these transmitters, the phase rotation does not matter because there are no actual 3-phase loads in the transmitter.
Inventory control, Home Depot Reisterstown Road, Baltimore, MD
I purchased this GPS Disciplined Oscillator a few weeks ago. The reason being, I wanted to make sure that this frequency counter was accurate.
Hewlett Packard 5315A Universal Counter
This Hewlett Packard 5315A was last calibrated in 1990. That made me suspicious. While I could send it back to Agilent and have it recalibrated, I thought it might be interesting to check it with a known standard.
When I connected the frequency counter to the 10 MHz GPSDO, it was -2.1 Hz off. At first, I thought perhaps the GPSDO was off; however, the spec for the LBE-1420 is 1 x 10-12 with a resolution of 1 Micro Hz. I let the HP unit warm up for 3 hours thinking maybe it was cold and would come back in tolerance. Nope, the frequency stayed about 2 Hz low.
It took about five seconds to find the full service manual online, which gives the alignment and calibration procedure in detail.
The first step is to use a DVM and check the +3, +5, and -5.2 power supplies. If they are off then adjust each accordingly. The next step is to check the +5 VDC pin on the Option 4 OCXO module and adjust as needed.
‘scope lead connected to reference oscillator pin
The calibration procedure for the HP 5315A is to connect a known 10 MHz reference to one channel of an oscilloscope and the output of the frequency counter OCXO to the other channel and look for slippage of the two signals. If the counter is on frequency, there should be no movement between the two waveforms. This is more accurate than trying to adjust the counter while looking at the frequency display on the counter.
Frequency alignment HP 5315A, Yellow squarewave trace LBE-1420 GPSDO, Violet sinewave trace 5315A reference oscillator
When I first connected it, the HP’s waveform was running backward at a pretty good clip. I adjusted the OCXO until there was no movement relative to the two waveforms. I let it sit like this for about three more hours before buttoning the HP unit back up. I am confident that the frequency counter is accurate +/- 0.3 Hz, which is good enough for my purposes.
LBE-1420 GUI
What I like about this Leo Bodnar GPSDO is that you can change the output frequency to any value between 1 Hz and 1.4 GHz. The output level is +13 dBm (per data sheet) with low phase noise, making it an excellent portable signal generator. The output is a squarewave, however, installing an LC type bandpass filter such as a Mini-Circuits SBP 10.7+ will round that out into a nice sinewave.
The Leo Bodnar website has a portable Windows executable for download, which can be used to program the output frequency and monitor performance.
I measured the output with my precision power meter; at 10 MHz it was +10.35 dBm. The low power output setting is about +5 dBm.
WWV carrier measurement with LBE-1420 as external 10 MHz reference
Another use for the LBE-1420 is as an external 10 MHz reference for test equipment. My Network Analyzer (and many other pieces of test gear) has an external 10 MHz input and if I use the spectrum analyzer to measure carrier or pilot frequency, it is nice to know that the test equipment is exactly on. I confirmed this by measuring the WWV carrier with my Siglent SVA-1032X spectrum analyzer using a long wire antenna.
Inexpensive Chinese GPSDO
Continuing with this interesting topic, I purchased a fairly cheap version from Ebay for further research. This particular unit is a clone of a BG7TBL, which is itself a clone. The interesting thing about these units is that they are using recycled OCXOs, which appear to be from decommissioned telecom equipment.
BG7TBL GPSDO block diagram
This diagram shows how these units work. The GPS signal is received by the GPS module, in this case, a uBlox M-7. The NMEA sentences and 1PPS are fed into the CPLD (Complex Programmable Logic Device). The NMEA sentences are also available on the RS-232 DB-9 connector.
GPSDO component side, GPS module lower left
The CPLD takes the output of the OCXO, in this case, a CTI OC`12SC38A, and compares the 1PPS from the GPS module to the 10 MHz from the OCXO module and adjusts the OCXO module by varying the voltage on the frequency adjust pin to keep it on 10 MHZ. It then sends the corrected 10 MHz and 1PPS signal out to BNC jacks. I found the 10 Mhz output level was +13.58 dBm as measured with my precison power meter. There is a built in bandpass filter, so the output is a good looking sinewave.
Judging by the CTI model number, it was made before 2015. There should be a date code on the bottom of the unit, but I did not feel like unsoldering it.
GPSDO OCXO
The one issue with this; OCXOs frequency drifts over time and eventually it will be out of the adjustment range. A closer look at the circuit board shows that it will accept several different OCXO modules. These modules run about $40-60 US new and $10-15 US used.
If an OCXO is suspected of being out of adjustment, they can be measured using the osciliscope method noted above.
Recently, I installed this very nice GatesAir FAX60HD.
This project was for WPGC, Washington, DC. WPGC (Prince George’s County) is an Audacy station with a Hip-Hop and R&B format. I was listening to The Good Morning Show on my drive from the hotel to the transmitter site, and those guys were hilarious! It’s nice to hear a well-programmed radio station.
It is always fun to accept new and interesting challenges. This is, to date, the largest transmitter either AM or FM that I have ever installed. Previously, I installed several FLX-40 units, which is quite a bit of power for the FM side of things.
MSC unit with touch screen pad controls both transmitters and exciters
This transmitter combines two FAX30 transmitters and is controlled by an MSC unit. The content stream for HD comes from an FMXi4g, which has several great features.
BDI inline watt meter
This station’s TPO is 45.7 KW with the HD carriers at -14dBc. While this is a class B station with an ERP of 50 KW, the four-bay half wave-spaced antenna requires a lot of wattage to make that TPO. This is a largely residential neighborhood, which is, I surmise, the reason for the half wave-spaced antenna.
WPGC main and backup antennas, Capitol Heights, Maryland
I was told that this is not the greatest part of town. The station has had some theft of outdoor air conditioner equipment in the recent past. That being said, it is much nicer than many areas we normally work in the NYC metro area. The transmitter site has been here since the station signed on in 1948.
WPGC transmitter siteFAX60HD power supplies and power amps installed
It took a bit of time to install the 42 power supplies and 48 power amps. The power amps were installed in the same slots as during the factory test cycle. Thus the data on the test sheet matches the data seen on the transmitter GUI when we turn it on.
FAX60HD cabinet interconnects completed
All of the cabinet interconnects; RF plumbing, grounding, AC supply, sample lines, and various control lines were completed.
Transmitter hybrid combiner for the two FAX30 transmittersFAX60HD, WPGC-FM Washington4 Inch Dielectric coax switch with 60 KW loadWPGC 4th harmonic
Most of the harmonics (2-10) looked like this. However…
WPGC 5th harmonic
This is something interesting that came up during the proof. When measuring the harmonics, most of them were in the -130 dB range. This one is slightly higher than that, which is due to the proximity of WFDC-DT on channel 15 (476-482 MHz), 1000 KW ERP about 10.6 miles away. Their signal was coming back down the transmission line from the FM antenna. This is a good demonstration of how other unwanted signals can get into the final sections of transmitters which can cause intermodulation mixing products. In this case, the FAX60 has several low-pass filters that remove this and other signals before that happens.
This is replacing a pair of combined BE FM25-T transmitters that were getting a little bit long in the tooth. The air staff has commented on the noticeable improvement of the station’s sound. The downside of tube transmitters is the delicate tuning procedure to reduce the AM noise. High-powered transmitter tubes are also getting more expensive and, for some types, harder to source.
For several months now, I have been taking care of an LPTV station in Syracuse and the owner is looking to sell. It is in a pretty good location; on a tall building downtown. It has a newer transmitter and antenna, EAS gear, located in a conditioned space, etc.
State Tower Building viewed from Montgomery and Water Street. Syracuse, New York
If you are interested, contact me for details: info (at) engineeringradio (dot) us
