Introduction
This article looks at various features and specifications, which are important to consider before choosing a 87-108 MHz FM tuner for long distance DX reception.
There are generally two distinct groups interested in long distance reception (FM DX). The first group listens to FM radio for entertainment purposes only. Their desire to receive long distance deep fringe reception is usually because they live in a relatively remote rural or country area. Sometimes they may live in a city area, and their favourite stations are located in a country town. Because the nearest FM stations are at least 100 miles away, a sensitive and selective FM tuner is needed for capturing weak signals.
The second group is not usually interested in receiving distant FM stations for entertainment purposes. They are called DXers (long-distance reception enthusiasts). DXers try to receive any distant FM station, no matter how weak and distorted they may be.
Second-hand FM tuners
Generally, there are very few current model FM tuners that are suitable for FM DX. As of 2006, most HI FI shops mostly sell receivers
(tuner and amplifier in one unit). Receivers are never equal to a
dedicated FM tuner, in terms of selectivity, sensitivity and image
rejection.
When I first looked at buying a FM tuner, several
USA HI FI annual equipment magazines were purchased. The annual Stereo Review magazine was
especially helpful because it contained component comparisons and
specifications for most of the then (1987) FM tuners. I also had to
learn how to correctly interpret FM tuner specifications. I eventually
decided on a Onkyo T-4087. This tuner had good selectivity, but the
power transformer always seemed to cause problems (this is not generally noted with other T-4087 tuners). Onkyo offered a
refund or credit for another tuner. I selected the AU$1,500 Onkyo Integra T-9090 II, which has proved to be a superior "DX machine".
Sensitivity Sensitive FM tuners generally feature Mosfet transistors in the first RF (radio frequency) stage. Mosfet transistors feature low noise and high sensitivity for weak signals. Typical RF noise figures range 2-3 dB for quality sensitive FM tuners that employ Mosfets in the first RF stage.
Very few FM tuners feature a GaAsFET RF front-end. The Kenwood KT-6040 is one tuner that includes a Toshiba 3SK121 GaAsFET.
Sensitivity ratings are usually expressed in microvolts (uV), and the lower the number the better.
Another rating scale sometimes used is the femtowatt (dBf). This indicates a tuner's sensitivity,
regardless of its input impedance (usually 75 or 300ohms). In most tuner brochures you will find the IHF (Institute of High Fidelity) sensitivity rating,
which is for weak mono signals. This corresponds to a -30-dB level of total harmonic distortion plus noise
(THD+N), that is a mono signal with 3% distortion. The IHF mono sensitivity specification is also called
useable sensitivity, which means the weakest signal most non-DXers would be prepared to tolerate. IHF
useable sensitivity usually ranges from 0.6uV (6.7dBf) to 1uV (11.2dBf). The lower the figure the better. A properly aligned KT-6040 tuner modified with 80 KHz filters is capable of useable sensitivity figures down to 0.2-0.4uV.
A tuner's ability to receive weak stereo signals is indicated by the IHF usable stereo sensitivity figure,
which usually ranges from 2uV (11.2dBf) to around 4uV (17dBf). The 50dB quieting level will indicate how strong a DX signal has to be to reach hiss-free level, and
that is often more important to those who like high fidelity FM DX. At 50dB stereo signal-to-noise-ratio, the tuner's hiss
is only 0.3% of the audio, and in a good FM tuner, that will be obtained with no more than 17uV (36.1dBf). The
50dB mono quieting specification is usually no more than 1.7uV with a good tuner.
By using a low 2dB noise figure tuneable Mosfet FM preamplifier, it is possible to achieve state-of-the-art RF sensitivity. The pre-amplifier can used indoors, providing the coax cable line loss is under 2dB. For rural or semi-rural areas, masthead mounting of preamplifiers may provide increased weak signal sensitivity in low-noise areas.
If you find that the sensitivity of your FM tuner is not improved by the use of a preamplifier, the external noise levels are either fairly high, or your FM tuner's noise level is sufficiently low, hence external noise becomes the limiting factor. Most of the best high sensitivity tuners (KT-6040, T-9090 II, T-4970) are sensitive enough that they do not benefit from using preamplifiers in strong signal environment city areas.
All sensitive FM tuners are able to detect FM signal meteor scatter "pings" on a daily basis. This of course assumes that the tuner is connected to an efficient high gain external directional high gain FM antenna. If meteor scatter pings can't be heard on vacant channels, the tuner's RF sensitivity is not good enough for FM DX.
Selectivity Selectivity refers to a tuner's ability to receive a weak station in the presence of stronger stations at
slightly lower or higher frequencies. "Alternate" selectivity refers to the tuner's ability to suppress interference from signals plus and minus
400kHz away from the tuned frequency. A rating of 80dB is good; while 100dB is excellent. "Adjacent" selectivity refers to suppression of signals plus and minus 200kHz away from the tuned
frequency, and is usually no better than 50dB on a unmodified FM tuner featuring narrow IF bandwidth. One example of a tuner with good selectivity is the
Onkyo T-9090 II FM tuner, which will suppress signals
plus and minus 300kHz away from the desired station by a factor of 10,000 (80dB). Signals plus and minus 400kHz away from the desired station are suppressed by a factor of 56,200 (95dB). These figures indicate excellent shape factor selectivity, considering the -3dB point is 150kHz. High selectivity is achieved by using several 150kHz ceramic filters in series in the 10.7 MHz IF strip. Some DX enthusiasts have replaced the stock filters with narrower MURATA 110kHz filters (part no: SFE10.7MHY-A). Recommended 10.7 MHz IF filters for enhanced FM tuner selectivity performance: Murata SFE10.7MT: 3db bandwidth ~ 80 KHz, 20dB bandwidth typically 160 KHz, insertion loss 6.5 +/- 2.5dB.
Murata SFE10.7MHY-A: 3db bandwidth 110 KHz +/- 30 KHz, 20dB bandwidth typically 260 KHz, insertion loss 7 +/- 2dB. My Onkyo T-9090 II tuner has been fitted with four 110 KHz Murata filters (selected by using the medium selectivity switch). The selectivity is now higher than 80 dB +/- 300 KHz. I have left the original five Murata 150 KHz filters in the tuner.
It should be noted that 110 KHz bandwidth results in slightly degraded audio fidelity compared to 150 KHz bandwidth. DXers are the only group likely to use 110 KHz IF filters. Most listeners will be content with 150 KHz bandwidth, especially since they are after HI FI audio quality.
Certain ceramic IF filters are designed for high selectivity AND fidelity. The Murata SFE10.7MZ2 is such a filter. The GDT (group delay time) specifications for the SFE10.7MZ2 filters are very good. This explains why Onkyo use the SFE10.7MZ2 150 KHz filters in their Integra series FM tuners.
Generally speaking, providing a weak FM signal doesn't suffer from adjacent channel interference, SFE10.7MZ2 150 KHz filters provide slightly better intelligibility compared to SFE10.7MHY 110 KHz filters.
Ceramic filters are used in the 10.7 MHz IF strip of FM tuners. They
are connected in series, and the number of filters used determines
the passband, shape factor, and stopband attenuation. For example, my
Onkyo T-9090 II FM tuner has 3 IF bandwidth settings (wide, narrow,
and super narrow). Five 150 KHz filters are switched in when 'super
narrow' is selected. Four 110 KHz filters are switched in
when 'narrow' is selected. Two 110 KHz filters are switched in
when 'wide' is selected.
I have compared the selectivity obtained by using the above different
filter configurations on my T-9090 II. Four 110 KHz filters gives the
highest overall selectivity. Five 150 KHz filters gives the second
best selectivity, followed by two 110 KHz filters, which is the
poorest.
Providing your FM tuner has sufficient IF gain, five 110 KHz filters
connected in series, gives the optimum selectivity performance. Five
150 KHz filters connected in series gives the optimum selectivity /
fidelity performance. By using more than six filters, selectivity will likely not offer any more noticeable improvement (law of diminishing returns). Also, extra unwanted IF noise is introduced, because of extra IC amplifier (buffer) stages, needed for extra filters.
Stopband attenuation performance
Stopband attenuation performance is largely a function of front-end selectivity, mixer performance, and filter stopband (dB).
The Onkyo T-9090 II features excellent stopband response of +/- 400 KHz @ 95 dB. In addition to the high varactor-tuned RF selectivity, this figure is largely obtained by four SFE10.7MZ2 150 KHz filters, and one SFE10.7MJ 150 KHz filter. The T-9090 II +/- 300 KHz selectivity is rated as 80 dB, while the +/- 200 KHz selectivity is rated as 45 dB.
The Murata SFT10.7MS3 has an indicated 60 dB spurious rejection response over 9 - 12 MHz IF. The SFT10.7MS3 stopband attenuation is rated as +/- 275 KHz @ 40 dB, while the 3 dB bandwidth is 180 KHz +/- 40 KHz, or ~ +/- 90 KHz.
The SFE10.7MS3 offers superior stopband attenuation. This offers one the advantage of improved selectivity rejection for signals greater than +/- 300 KHz of the wanted DX signal. However, the 10.7MS3 will not improve +/- 100 or 200 KHz tuner selectivity. Instead, the 10.7MS3 offers improved tuner image rejection when listening in a city area with several strong local signals.
Assuming two 110 SFE10.7MHY and two SFE10.7MS3 filters were installed, it would be appropriate to install the MS3 filters last in the chain. This arrangement would provide the best selectivity / stopband attenuation combination.
When one searches the Internet, you will find that most DXers do not install filters with a 3 dB bandwidth greater than 150 KHz (+/- 75 KHz). Serious DXers tend to use tuners that include 150 KHz filters for "hi fidelity" DX, 110 KHz for medium, and even 80 KHz for super narrow. For the reasons stated above, I generally don't recommend filters greater than 150 KHz for DX work. My preferred approach is as follows:
* Modified Onkyo T-9090 II (T-9990) with five 150 KHz Murata filters (high fidelity DX), and four 110 KHz filters (difficult weak signals 200 KHz adjacent to locals). Two A/B RF inputs are connected to two Triax FM-8 yagis. It should be noted that other Onkyo tuners such as the T-488F, T-4970, and T-4711 are also suitable for "HI FI" DX. The T-9090 II is my general listening DX tuner for Sporadic E and enhanced tropospheric openings.
* Modified Kenwood KT-6040 with three 110 KHz Murata filters (medium selectivity), and two 110 KHz and four 80 KHz filters for extremely weak DX (150-300 mile daily troposcatter / 400 mile aircraft scatter, etc). The KT-6040 is my preferred choice for determining the extreme limits of troposcatter and aircraft scatter weak signal propagation.
FM tuners featuring switchable wide or narrow IF bandwidth will have superior selectivity. This is a
highly desirable feature for successful FM DXing. Strong signal handling and spurious rejection Many cheaper FM tuners will suffer from overload and spurious images of local stations all over the dial,
particularly in city areas. This is often because the tuner's RF amplifier is a bipolar transistor type and
has no pre-selection. To achieve overload immunity, a tuner should have a MOSFET RF amplifier front end,
tuned with varicap diodes. Dual gate MOSFETs are excellent because they operate linearly over a wide
range of signal strengths and also offer a low noise figure, hence feature good sensitivity.
Variable capacitance (varicap) tuned RF front
ends act as an automatic preselector, which only allows a narrow window passband of signals to the
RF amplifier. Varicap front ends offer superior spurious rejection of unwanted crossmodulation and overload. The image spurious rejection ratio should be 90dB or more. Some examples of FM tuners with state of the art image rejection are the Onkyo T-9090 11, Onkyo T-4711, Yamaha T-85, and Sony SASE5S. All these FM tuners have a image rejection ratio of 100dB.
The Onkyo T-9090 II and Yamaha T-85 both employ front end a dual-stage Mosfet designs that can
handle even extremely high input signals without cross modulation
problems. Six stage varactor tuned diodes also greatly reduce the
input bandwidth before the first RF amplifier. This prevents
intermodulation distortion and other undesirable side effects, which
can occur if a strong signal is amplified directly.
FM tuner frequency increments Many cheaper tuners only tune in 100kHz steps, which makes it impossible to slightly de-tune from a local
station to minimize interference to a DX signal. Look for a tuner that features 50KHz, or even better, 25kHz steps which
has 3 digits after the decimal point (99.925, etc). Recommended high performance tuners that feature 25 KHz tuning steps, include the Onkyo T-9090 II, Onkyo T-4711, T-4670, T-4700, T-4970, T-488F, Kenwood KT-6040, and Yamaha T-85.
Capture ratio The capture ratio indicates the tuner's skill at locking on to one signal and ignoring others (including
reflections) at the same frequency. A tuner with a good
capture ratio of 1dB or less could also improve results, when a phasing unit is used to eliminate a local station
on the same frequency. Miscellaneous features and specifications Dual antenna 75ohm inputs are useful when two separate DX yagi antennas are used, allowing the
possibility for instant comparisons. A center tuned indicator is very useful, particularly on tuners with
25kHz tuning increments. Certain Fiji and New Caledonia FM transmitters use unusual frequencies (97.425MHz). In this case a center-tuned indicator is ideal for tuning accuracy. A digital strength meter
or bar graph is useful for antenna orientation and relative strengths of DX signals. Radio Data Service (RDS) is carried on an extra sub-carrier of the main signal. The RDS information
includes a station identifier that can be displayed along with other information. A very
small number of stations in Australia use RDS, mainly the JJJ-FM network. For RDS information to
work properly, the FM signal has to be strong, hence is not ideal for FM DX.
Other specifications such as AM rejection, IF rejection, THD (Total Harmonic Distortion) and maximum
S/N are usually adequate on most FM tuners, and hence are usually adequate for successful
FM DX.
The best portable radio, which has been tested by Australian FM DXers, is the Panasonic RF-B65. This radio was found to have superior or near equal performance on FM, to the Grungig Satellit 700. Using just it's own whip antenna, the FR-B65 was able to receive Indonesian FM from Australia at a distance of 850 miles. Is the Icom R-7000 scanner useful for FM DX?
When I first tried my unmodified R7000 for FMDX I was
disappointed by the FM wide selectivity. The R7000 only had only two Murata ceramic filters;
(1) 230 KHz, and (1) 150 KHz. With this arrangement, I
found that the adjacent (+/-200 KHz) selectivity was
poor.
I replaced the stock filters with two 110 KHz Toko
ceramic filters. The selectivity was much better, and
I also found that tuning +/- 200 KHz of local signals
produced mainly white noise.
The only disadvantage was 1-2 dB reduced sensitivity.
This is because the Toko filters had slightly higher insertion
loss. This did not concern me because I use a tuneable
88-108 MHz Mosfet pre-amp. The pre-amp also partially
compensates for the R7000's un-tuned wide RF front-end.
In city areas, you will find several images of local
stations across the dial. A tuned pre-amp, or bandpass
filter will improve the R7000's image and overload
problems.
My next project is to replace the two Toko 110 filters
with Murata SFE10.7MHy-A 110KHz filters. These filters
have lower loss, and improved shape factor.
However, I'm still impressed with the 110 KHz Toko filters.
Murata 80 KHz, 110 KHz, and 150 KHz filters are currently available in small quantities from the FM and TV DX plaza.
I once compared my modified R7000 to the TEAC T-515 FM tuner
(60dB +/- 300 KHz selectivity). The R7000 was better,
though not as good as my Onkyo T-9090 11 (80 dB +/-
300 KHz). I would estimate that the R7000, with two
110 KHz filters has a selectivity spec of ~ 70dB +/-
300 KHz.
Alignment of FM tuner's RF and IF stages
It has been noticed by myself that certain FM tuners are often not aligned for optimum performance. This was certainly true in the case of my Onkyo T-9090 II tuner.
I have aligned several FM tuners. I use a tunable simple signal
generator (Tony Mann design), which produces a weak stable signal. This signal source is not directly connected to the antenna input, rather it is used on the other side of the house! I
need to make sure the signal disappears once the antenna is removed.
Before attempting alignment, you will need the tuner
service manual for step by step instructions on the procedure. If you
do it yourself, plastic alignment tools are essential. Don't try
using a metal screwdriver!
Conclusion To review what FM tuners are available, look at the annual HI-FI equipment magazines, which feature
current FM tuners and specifications. Australia HI-FI and Stereo Review HI-FI equipment magazines are available at newsagents, HI-FI stores and libraries.
Visit every HI FI shop in your area. Place wanted ads in these shops.
Look through all the local classified ads in your local newspapers.
If required, place a wanted ad in these papers.
Excellent FM tuners have been made during the past
ten years, so it might be worthwhile considering second hand equipment in the Trading Post, etc. Ebay is another source. List of recommended high-end FM tuners
* Kenwood KT-6040.
European area DXers generally use the following tuners:
* Kenwood KT-6040 Photo. Links
Murata SFE10.7MJA10: 3dB bandwidth 150 KHz +/- 30 KHz, 20dB bandwidth typically 290 KHz, insertion loss 4+/- 2dB.
Onkyo Integra series tuners are especially suited for filter modifications. For example, a Onkyo T-9090 11 or T-4711 tuner modified with 110 KHz filters.
Circuit example of RF front-end with varacter diodes, as used in the Onkyo T-9090 II.
One portable radio that has received good reports from European FM DXers, is the Grundig Satellit 700. Some DXers have modified this radio with Murata 110 KHz filters.
* Onkyo T-9090 II Review.
* Onkyo T-4711.
* Onkyo T-488F (with DYNAS high selectivity function).
* Yamaha T-85.
* Denon TU 1500RD (still in current production).
* Sony ST-SA5ES. Review.
* Onkyo T-9990 (European equal of the T-9090 II).
* Onkyo T-4711.
* Onkyo T-4970 (European equal of the T-488F).
* Sony ST-SA5ES.
Your Guide to FM Tuners with DX Potential. USA web page.Ceramic filter modifications for improving selectivity. From the AMFMDX web site.
Tuner information center. USA web page.FM Tuner Specifications. . What do they actually mean?. Fanfare USA web page.Where can FM DXers purchase Murata SFE10.7 MHY-A, 110 KHz IF filters.
Low noise Mosfet pre-amp for FM DX.
Onkyo T-4711 IF filter modifications.
Copyright © 2006 Todd Emslie
